I have a GM 7.5/7.625 10-bolt limited slip differential in my 1981 Chevy Monte Carlo and I am in the process of swapping out the differential cover and replacing the gear oil. I have a Gen I Chevy small block 350 with a TH350 transmission in the car and the previous owner used to drag race the car but I do not. I have done a bunch of research and have decided to go with an 80W-90 mineral gear oil. I went to the local Chevy dealership and purchased the ACDelco Limited Slip Axle Lubricant Additive and while I was there I also bought the ACDelco 80W-90 Axle Lubricant. This is what I planned on using but it only meets the GL-4 requirements. While this gear oil has been formulated for GM axles I am concerned that it will not provided the same shock protection as a GL-5 rated gear oil. While I do not drag race the car I do drive it hard at times and since it is geared for the strip it will regularly break the rear wheels loose when shifting from 1st to 2nd and from 2nd to 3rd gear. The downside to the GL-5 would be the concern about EP additives and their effect on my older components. I know that most mineral gear oils now contain "inactive" sulfur which is mediated by metal deactivators and which do not allow the sulfur to interact with the copper-alloy metals I still wonder if I would be better off with the GL-4 ACDelco gear oil or if I can safely upgrade to the GL-4/GL-5 rated to get the EP protection. If I go that route was going to use Valvoline’s High Performance 80W-90 gear oil. Thoughts? Suggestions?
Gear Oil
- Thread starter VintageMonte
- Start date
- Status
You might give RedLine a call. They make gear oils that are designed to handle shock and protect gears.
Thanks OneEyeJack. These were good reads but they both point me back to synthetic gear oils. However, Eaton (the likely manufacturer of my GM differential) still specs mineral oil for their differentials. Also, I have a hard time trusting any testing conducted by a manufacturer when their own product out-performs every other product tested by such a larger margin. I have seen a lot of negative content in other forums about Amsoil synthetic gear oils. Does anyone else have experience with it?
In the '80s, Saginaw made your Monte Carlo's rear axle. I don't think Borg Warner built any for the Monte Carlos as they did for the Camaros in that era. That Borg Warner was an oddball. In the '90s, GM's Saginaw Axle Division was spun off into AAM (American Axle).
There is no inherent reason for mineral over syn. If it's specifically recommend over syn, it's because of the limited slip. It's possible and likely your Saginaw has an Eaton Posi limited slip. If so, the mineral oil is recommended for that reason and Eaton still specs that.
From the Current Eaton Posi FAQ:
"Eaton Posi
What kind of oil should I use? Can I use synthetics? Do I need a friction additive/modifier?
Eaton Posi units perform best when using GL4 (or better) mineral/petroleum based gear oil. A four ounce bottle of friction additive/modifier is also necessary for optimum performance."
The performance of the limited slip (the bias ratio, or how much torque it can transfer from the low traction side to the high traction side) is calculated based on the friction characteristics of a mineral oil with the right amount of FM. Does that mean you can't run syn? IMO, no, but it's an experiment. The friction characteristics are different for syn vs mineral oil so you might lose some bias ratio in the limited slip and/or you could end up with chatter. If you want to try, be prepared for a quick oil change if it doesn't work out but give it at least 100 miles and right after installing the new oil, find a parking lot and do some figure 8s under some power to circulate it into the clutch packs. This is a good thing to do with any LS after an oil change anyway.
The rest of the diff doesn't care. That part of it probably would benefit from a syn oil. If you want to stay in the mineral realm, Chevron ESI oils are among the best in that genre.
There is no inherent reason for mineral over syn. If it's specifically recommend over syn, it's because of the limited slip. It's possible and likely your Saginaw has an Eaton Posi limited slip. If so, the mineral oil is recommended for that reason and Eaton still specs that.
From the Current Eaton Posi FAQ:
"Eaton Posi
What kind of oil should I use? Can I use synthetics? Do I need a friction additive/modifier?
Eaton Posi units perform best when using GL4 (or better) mineral/petroleum based gear oil. A four ounce bottle of friction additive/modifier is also necessary for optimum performance."
The performance of the limited slip (the bias ratio, or how much torque it can transfer from the low traction side to the high traction side) is calculated based on the friction characteristics of a mineral oil with the right amount of FM. Does that mean you can't run syn? IMO, no, but it's an experiment. The friction characteristics are different for syn vs mineral oil so you might lose some bias ratio in the limited slip and/or you could end up with chatter. If you want to try, be prepared for a quick oil change if it doesn't work out but give it at least 100 miles and right after installing the new oil, find a parking lot and do some figure 8s under some power to circulate it into the clutch packs. This is a good thing to do with any LS after an oil change anyway.
The rest of the diff doesn't care. That part of it probably would benefit from a syn oil. If you want to stay in the mineral realm, Chevron ESI oils are among the best in that genre.
Last edited:
Originally Posted By: VintageMonte
Thanks OneEyeJack. These were good reads but they both point me back to synthetic gear oils. However, Eaton (the likely manufacturer of my GM differential) still specs mineral oil for their differentials. Also, I have a hard time trusting any testing conducted by a manufacturer when their own product out-performs every other product tested by such a larger margin. I have seen a lot of negative content in other forums about Amsoil synthetic gear oils. Does anyone else have experience with it?
I would try some Rotella 85w140 dino gear lube in that diff if you don't want to use synthetic. BelRay makes dino gear lubes that are excellent as well.
http://www.shell.com/rotella/product-selector.html
Thanks OneEyeJack. These were good reads but they both point me back to synthetic gear oils. However, Eaton (the likely manufacturer of my GM differential) still specs mineral oil for their differentials. Also, I have a hard time trusting any testing conducted by a manufacturer when their own product out-performs every other product tested by such a larger margin. I have seen a lot of negative content in other forums about Amsoil synthetic gear oils. Does anyone else have experience with it?
I would try some Rotella 85w140 dino gear lube in that diff if you don't want to use synthetic. BelRay makes dino gear lubes that are excellent as well.
http://www.shell.com/rotella/product-selector.html
Last edited:
Thank you Jim. The rear gear has been changed out in my Monte Carlo and I believe it is an Eaton posi which is why I was relying on their specs.
I also am not too interested in the synthetic because of the potential for chatter due to the different friction characteristics and because I know that only mineral oil has ever been used in the differential.
I was all prepared to go with the ACDelco 80W-90 gear oil and the additional posi additive until I saw that it was only GL-4. I would prefer to have a gear oil that has the shock protection found in the GL-5 rated oils. Is that thinking right?
I also am not too interested in the synthetic because of the potential for chatter due to the different friction characteristics and because I know that only mineral oil has ever been used in the differential.
I was all prepared to go with the ACDelco 80W-90 gear oil and the additional posi additive until I saw that it was only GL-4. I would prefer to have a gear oil that has the shock protection found in the GL-5 rated oils. Is that thinking right?
Chevron ESi is GL-5 but its hard to find in some areas. You might also look at Valvoline High Performance 75W90 or 80W90, which is commonly available here (also, the Napa grand is exactly the same stuff). I also have a LS that requires mineral gear oil but I was looking for one with better cold temp specs than the Chevron 80W90 I had in there (I'm in Ohio) and recently changed to the Valvoline HP 75W90. It has the limited slip additive already added, so I didn't add any more and it's working fine with my Auburn Hp LS. I have some additive in case it begins to chatter... but after about 600 miles, I doubt it will.
FYI, most of the Borg Warner and some of the Saginaw axles of the '80s had a "Borg Warner" limited slip, which used cone clutches instead of plate clutches and is almost exactly the same unit as the Auburn HP I am running now. Not that it matters, since they also want a mineral oil.
Do you know for sure what you have? I.e. you installed the unit or you can tell them apart? If you took a pic with the cover off, I could tell you if you don't know.
FYI, most of the Borg Warner and some of the Saginaw axles of the '80s had a "Borg Warner" limited slip, which used cone clutches instead of plate clutches and is almost exactly the same unit as the Auburn HP I am running now. Not that it matters, since they also want a mineral oil.
Do you know for sure what you have? I.e. you installed the unit or you can tell them apart? If you took a pic with the cover off, I could tell you if you don't know.
I had settled in on the Valvoline 80W-90 after I realized the ACDelco was only GL-4. I live in western PA but I am not concerned about cold weather performance because the car goes in storage for the winter. I am not sure what posi gear is in the car because I have never had the cover off. I was waiting until I sorted out the gear oil before I started the process of removing the cover but I am pretty confident it is something made for GM as the original owner that drag raced the car only had GM cars. I will know soon enough but as you mentioned, they all require mineral oil so it is more about choosing the best of that bunch. I am hesitant to use the Valvoline now after reading this study posted earlier in the forum:
http://www.lastgreatroadtrip.com/wp-content/uploads/2010/11/gear-oil-comparison.pdf
What are your thoughts on these findings?
Wouldn't the Valvoline at GL-4 / GL-5 rating be better then ACDelco at just GL-4 even though it is "formulated for GM axles"?
http://www.lastgreatroadtrip.com/wp-content/uploads/2010/11/gear-oil-comparison.pdf
What are your thoughts on these findings?
Wouldn't the Valvoline at GL-4 / GL-5 rating be better then ACDelco at just GL-4 even though it is "formulated for GM axles"?
Isn't GL-5 for sliding protection, not shock protection? GL-5 standards are for hypoid gears where there is a lot of sliding action of the gear teeth. (Hypoid means that the input shaft is on a different plane...usually lower in a differential...than the output shaft.) I don't know what shock protection means in gears. Is it just marketing lingo? Or is there actual excessive shock between gear teeth? Wouldn't that be mainly a feature of the film strength of the oil with higher viscosity, other things remaining equal, giving higher film strength? And, PAO offer higher film strength than petro including Group III "synthetic".
I see this about the AC Delco 80W-90 GL-4 rear axle lube: "Suitable for GM applications requiring GM spec 9985290." Shouldn't that read, "Suitable for only...9985290."? I would never use GL-4 gear oil in any differential that didn't specifically call for it.
I see this about the AC Delco 80W-90 GL-4 rear axle lube: "Suitable for GM applications requiring GM spec 9985290." Shouldn't that read, "Suitable for only...9985290."? I would never use GL-4 gear oil in any differential that didn't specifically call for it.
This is what I have on GL ratings from an old thread posted on this forum years ago:
GL-4; Specified for hypoid gear service under severe service but without shock loading. This classification is essentially obsolete but is still specified by some manual transmission/transaxle manufacturers. Implies an EP/AW additive package that contains 30% to 50% less S-P additives than the GL-5 service classification. Some Marine Gear Lubes fall into this classification, especially the full Synthetic Marine Gear lubes and specialty blenders MT lubes that use high levels of esters.
GL-5; Specified for hypoid gear service but with shock loads and severe service operation. Usually meets Mil-L-2105D and in most cases, is the multipurpose automotive gear oil. Most 75W90 to 75W140 grades meet the GL-5 classification. This grade has a high level of Extreme-Pressure additives that could be mildly corrosive to nonferrous parts, such as brass, bronze and aluminum parts. Most of the modern GL-5 lubes contain metal deactivators that prevents attacks by the extreme-pressure additives. In addition to EP additives, these lubes contain rust inhibitors, defoamants, friction modifiers, thickeners, and Viscosity Index Improvers.
I am only interested in using the ACDelco gear oil that is GL-4 because I have a GM type differential. I would prefer to stick with mineral oil since all manufacturers that produced posi gears for GM recommend it. That said, I am trying to figure out it it is better to go with a GL-4 / GL-5 rated oil (Valvoline 80W-90) or stick with the ACDelco GL-4.
GL-4; Specified for hypoid gear service under severe service but without shock loading. This classification is essentially obsolete but is still specified by some manual transmission/transaxle manufacturers. Implies an EP/AW additive package that contains 30% to 50% less S-P additives than the GL-5 service classification. Some Marine Gear Lubes fall into this classification, especially the full Synthetic Marine Gear lubes and specialty blenders MT lubes that use high levels of esters.
GL-5; Specified for hypoid gear service but with shock loads and severe service operation. Usually meets Mil-L-2105D and in most cases, is the multipurpose automotive gear oil. Most 75W90 to 75W140 grades meet the GL-5 classification. This grade has a high level of Extreme-Pressure additives that could be mildly corrosive to nonferrous parts, such as brass, bronze and aluminum parts. Most of the modern GL-5 lubes contain metal deactivators that prevents attacks by the extreme-pressure additives. In addition to EP additives, these lubes contain rust inhibitors, defoamants, friction modifiers, thickeners, and Viscosity Index Improvers.
I am only interested in using the ACDelco gear oil that is GL-4 because I have a GM type differential. I would prefer to stick with mineral oil since all manufacturers that produced posi gears for GM recommend it. That said, I am trying to figure out it it is better to go with a GL-4 / GL-5 rated oil (Valvoline 80W-90) or stick with the ACDelco GL-4.
The Study:
While I greatly appreciate the study in many ways, you have to bear in mind several things. First, it's a sales tool for Amsoil and if they have the slightest opportunity to show the competition in a bad light, they will. And did. It doesn't say how many samples of the competition were tested. Did they run ten samples and one came out bad and that's the one they show, or did they only test one sample, was it just one sample tht unluckily came from a bad batch or is everything put out by that company bad. The last idea is highly unlikely and do you think that if Amsoil caught a bad batch in this test, would they have presented it? Again, highly unlikely. I don't think Amsoil would fudge the results but as I said, they would gleefully emphasize downsides in the other oils. Bottom line, weigh the evidence according to the fact that it's being used as a sales tool. I didn't let it stop me from buy the Valvoline.
As to the GL-4 v GL-5 thing, I think you are overanalyzing. Get GL-5 if you can because it has more EP additives as Ken2 alludes to above (sliding protection). "Shock" is as much a marketing term as a tribological one but the cushion effect for sudden loads comes as much from the viscosity as anything and a thicker oil delivers more of that while reducing drivetrain efficiency (thick oil costs HP). Overall, EP (Extreme Pressure) additives are what you really need in the end. The GL-4 spec was the old standard and was carried on for transmissions for a time because it had less deleterious effects on brass sychros. The more modern EP additives can do this as well, so GL-4 oil only is getting harder to find. Most new oils list GL-5 and are backspec'ed to GL-4. "Formulated for GM axles" is a red herring. Your axle was originally designed for GL-4 because that was the best spec in 1981. GL-5 is a higher spec lube for today. It's better... use it!
While I greatly appreciate the study in many ways, you have to bear in mind several things. First, it's a sales tool for Amsoil and if they have the slightest opportunity to show the competition in a bad light, they will. And did. It doesn't say how many samples of the competition were tested. Did they run ten samples and one came out bad and that's the one they show, or did they only test one sample, was it just one sample tht unluckily came from a bad batch or is everything put out by that company bad. The last idea is highly unlikely and do you think that if Amsoil caught a bad batch in this test, would they have presented it? Again, highly unlikely. I don't think Amsoil would fudge the results but as I said, they would gleefully emphasize downsides in the other oils. Bottom line, weigh the evidence according to the fact that it's being used as a sales tool. I didn't let it stop me from buy the Valvoline.
As to the GL-4 v GL-5 thing, I think you are overanalyzing. Get GL-5 if you can because it has more EP additives as Ken2 alludes to above (sliding protection). "Shock" is as much a marketing term as a tribological one but the cushion effect for sudden loads comes as much from the viscosity as anything and a thicker oil delivers more of that while reducing drivetrain efficiency (thick oil costs HP). Overall, EP (Extreme Pressure) additives are what you really need in the end. The GL-4 spec was the old standard and was carried on for transmissions for a time because it had less deleterious effects on brass sychros. The more modern EP additives can do this as well, so GL-4 oil only is getting harder to find. Most new oils list GL-5 and are backspec'ed to GL-4. "Formulated for GM axles" is a red herring. Your axle was originally designed for GL-4 because that was the best spec in 1981. GL-5 is a higher spec lube for today. It's better... use it!
I completely agree with you about the fact that the study was conducted and presented by Amsoil. I am not saying it is not "true" but it has to be viewed under the premise that the "best" oil is made by those doing the testing.
Thanks for your insight about the ACDelco oil. I figured it would be better to go ahead and use the Valvoline since it was GL-5 but I was holding back because of the fact that ACDelco was made for GM differentials. My only other reservation about going to the GL-5 was the corrosion factor but it sounds like that is not a concern any longer with differentials.
I know you did not use the posi additive (yet) on your most recent change and that the Valvoline has the additive built in but wouldn't it be best just to add it in to start with? What is the downside to adding it other then the cost?
Thanks for your insight about the ACDelco oil. I figured it would be better to go ahead and use the Valvoline since it was GL-5 but I was holding back because of the fact that ACDelco was made for GM differentials. My only other reservation about going to the GL-5 was the corrosion factor but it sounds like that is not a concern any longer with differentials.
I know you did not use the posi additive (yet) on your most recent change and that the Valvoline has the additive built in but wouldn't it be best just to add it in to start with? What is the downside to adding it other then the cost?
I agree with Jim on the Chevron ESI product.
IF I were forced to run a mineral based gear oil, it would be that, or the Torco RGO I would use (maybe the Lakewood BFL as well, if I could not find one of the others).
Your Eaton Posi has STEEL clutchpack plates if it is from that vintage (as opposed to carbon fiber in the newest/latest versions).
THAT could be why Eaton is soooo insistent on you using a mineral based oil even more adamantly than they do for the carbon fiber diffs.
IF I were forced to run a mineral based gear oil, it would be that, or the Torco RGO I would use (maybe the Lakewood BFL as well, if I could not find one of the others).
Your Eaton Posi has STEEL clutchpack plates if it is from that vintage (as opposed to carbon fiber in the newest/latest versions).
THAT could be why Eaton is soooo insistent on you using a mineral based oil even more adamantly than they do for the carbon fiber diffs.
Originally Posted By: VintageMonte
I know you did not use the posi additive (yet) on your most recent change and that the Valvoline has the additive built in but wouldn't it be best just to add it in to start with? What is the downside to adding it other then the cost?
The reason you don't what more LS additive (FM for friction modifier) is that it reduces the oxidative stability of the oil... it's ability to resist breakdown in high heat or long term operation. As I just said in another post, FM's have improved over the past decade and this is less an issue than it once was with only low-temp organic FMs. Since there is no advantage to having more than you need, I advise buying the oil with it installed and seeing how it goes. If you get chatter, add it in once ounce increments, doing some figure-8s in a parking lot, driving with it a little and then repeat until you reach the four ounce limit or chatter stops.
I know you did not use the posi additive (yet) on your most recent change and that the Valvoline has the additive built in but wouldn't it be best just to add it in to start with? What is the downside to adding it other then the cost?
The reason you don't what more LS additive (FM for friction modifier) is that it reduces the oxidative stability of the oil... it's ability to resist breakdown in high heat or long term operation. As I just said in another post, FM's have improved over the past decade and this is less an issue than it once was with only low-temp organic FMs. Since there is no advantage to having more than you need, I advise buying the oil with it installed and seeing how it goes. If you get chatter, add it in once ounce increments, doing some figure-8s in a parking lot, driving with it a little and then repeat until you reach the four ounce limit or chatter stops.
I wonder what the manual trany fill is on the Tacoma. My 2011 is the best shifting one
i have had so far. Even second to first down shifts. Jim do you live near Camp Perry?
The CMP NOrth Store is my favorite trip. The rack,s of m1,s for sale can make you crazy.
Cam you have too many?
i have had so far. Even second to first down shifts. Jim do you live near Camp Perry?
The CMP NOrth Store is my favorite trip. The rack,s of m1,s for sale can make you crazy.
Cam you have too many?
Thank you for all you insight Jim. I get where you are coming from on the additive and will wait and see how it goes on oil with the additive built in. I have settled in on the Valvoline High Performance 80W-90. I took an closer look at the Amsoil study and took out the scores for all of the oils for the cold weather tests since I never use my Monte in these conditions. I also took out price because it is irrelevant to the performance of the oil. When you do that for all 14 oils tested (synthetic and mineral) and re-tally the scores the Valvoline High Performance 80W-90 oil comes in 5th place which is great considering it is competing against synthetic oils. Add this into the fact that I can get it at my local NAPA store and it is the best choice for me.
Originally Posted By: VintageMonte
Thanks OneEyeJack. These were good reads but they both point me back to synthetic gear oils. However, Eaton (the likely manufacturer of my GM differential) still specs mineral oil for their differentials. Also, I have a hard time trusting any testing conducted by a manufacturer when their own product out-performs every other product tested by such a larger margin. I have seen a lot of negative content in other forums about Amsoil synthetic gear oils. Does anyone else have experience with it?
Been using Amsoil Severe Gear for years, it works as advertised.
Thanks OneEyeJack. These were good reads but they both point me back to synthetic gear oils. However, Eaton (the likely manufacturer of my GM differential) still specs mineral oil for their differentials. Also, I have a hard time trusting any testing conducted by a manufacturer when their own product out-performs every other product tested by such a larger margin. I have seen a lot of negative content in other forums about Amsoil synthetic gear oils. Does anyone else have experience with it?
Been using Amsoil Severe Gear for years, it works as advertised.
Originally Posted By: VintageMonte
I have seen a lot of negative content in other forums about Amsoil synthetic gear oils. Does anyone else have experience with it?
Really? What were the specific comments? What sites?
I have seen a lot of negative content in other forums about Amsoil synthetic gear oils. Does anyone else have experience with it?
Really? What were the specific comments? What sites?
I don't have specific sites to reference as I did not save them. I was surprised by the negative Amsoil comments but I did see them on two different sites and just thought I would ask here.
- Status