What harm is there in an oil that thins out?

Patman

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Technically speaking, what damage can be done if an oil thins out when in use? Let's just say for argument's sake that a particular engine likes an oil that is around 13cst at 100c. So you run an oil like GTX 10w40 and it starts out at around 14 cst and after 3000 miles it finishes up at around 12cst. So it's thinned out considerably, however it's still in the "sweet spot" of viscosity that this particular engine has been proven to like. So what is the problem with this scenario? Would this engine then have a tendancy to get more dirty due to the breaking down of the VII? Or is there really no harm in an oil thinning out as long as the viscosity is still within an acceptable range? One of the reasons I ask this is because of the recent rash of 0w40 results where they have thinned out, and also because of the recent LS1 analysis with GTX 10w40 where it thinned out a lot but the wear numbers were the best LS1 results I've seen to date.
 
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quote:
Originally posted by Patman: Technically speaking, what damage can be done if an oil thins out when in use? Let's just say for argument's sake that a particular engine likes an oil that is around 13cst at 100c. So you run an oil like GTX 10w40 and it starts out at around 14 cst[the rest deleted.....]..... One of the reasons I ask this is because of the recent rash of 0w40 results where they have thinned out, and also because of the recent LS1 analysis with GTX 10w40 where it thinned out a lot but the wear numbers were the best LS1 results I've seen to date.
As long as you have any reason to believe that TBN's are still O.K., it does not matter if it thins out a little. Assuming, of course, it is not New Orleans, Mexico, or Darwin Australia in Summer. For those extreme applications, I'd use monograde 30 in mineral or synthetic. From August 2000 to August 2001, I kept the same load of Castrol GTX 10W-40 in my engine. Never changed it. Excellent wear results. Of course, those 12 months, I ran up less than 3,000 miles in the old beater.
 
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Good question, But hasn't the M1 0w-40 been thinning out? I was thinking of trying this one. Wear rates have been good though and SS doesn't seem to be thinning out. [ January 24, 2003, 05:37 PM: Message edited by: buster ]
 
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Buster, thinning 0w40 providing good results was one of the basis of his question as I understood it. [ January 24, 2003, 06:29 PM: Message edited by: Giles ]
 

mdv

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I guess what I don't understand is that there must be some specific reason that you want to buy a 40 wt, as the vast majority of cars in NA spec 30 wts. Since you made the decision to buy a 40 wt., why would you accept the fact that it thins to a 30 wt. with use? Why not buy a good stable 30 wt. that's near the top of the range and have a more consistent viscosity??
 
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I think that this may explain some of the consumption issues we see when people say they're fine with an oil for the first 2-3k mi., and then the consumption starts. Guess the oil is breaking down/shearing down and the resultant viscosity decrease results in thinned oil which results in thermal instability and burn-off. As far as protection...I think you'd be OK if it's still roughtly in the same viscosity spec./range.
 
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Patman, good question. I was thinking the same thing as I was looking at the 0w40 analysis that showed it thinning out. I think the thinning might be a problem if the Viscosity Improvers are the kind that have little or now lubricity. But I think the 0w40 uses esters, which I think do have lubricity. Now if your vehicle is recommending 5w30, and you start out at a 0w40 but it thins out to a 30 weight, you still are within the specs of the engine. Maybe you can run the drain interval longer because it has more viscosity to lose before falling out of the grade recommended for the engine. I personally haven't seen a 0w40 M1 UOA report I didn't like as far as wear numbers go at any drain interval.
 

Patman

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Originally posted by mdv: I guess what I don't understand is that there must be some specific reason that you want to buy a 40 wt, as the vast majority of cars in NA spec 30 wts. Since you made the decision to buy a 40 wt., why would you accept the fact that it thins to a 30 wt. with use? Why not buy a good stable 30 wt. that's near the top of the range and have a more consistent viscosity??
I haven't decided on a 40wt for my car, I'm just curious about the topic. For my 95 LT1 Firebird, with close to 80k on it now I do believe an oil at the top end of the 30wt scale or lower 40 would offer me the best protection. Since I want to continue using Schaeffer oil, I'm going to mix in a couple of quarts of 15w40 to the rest being 10w30, and through analysis I'll see what ratio I need in order to get my viscosity to the desired 12-13cst range I'm seeking. By the way, even though most cars do specify 30wts, I believe this spec is valid when the engine is new and tight, but as it ages I do believe you should increase the viscosity slightly to get better wear. The oil companies know this too, which is why the high mileage oils are all formulated to be at the upper end of the viscosity range. This is another reason why many users report lower consumption with these oils.
 
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Personally I think it depends on the weight the engine was designed for and the hydrodynamic lubrication it can build. I have some analisis at this page that show specifically what can happen in terms of wear. http://american.netfirms.com/Analisis_de_Aceite/Perdida_de_Viscosidad/perdida_de_viscosidad.html Sorry, it is in spanish like most of the site, but as you will see from the 5 samples presented, as viscosity decreased, bearing wear increased. The las two sheared but filled with soot, rasing the apparent viscosity, but still showing excess wear. Lately I've been too busy to write in both languages.
 

Patman

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Originally posted by sprintman: Patman I just tried a 10W30 mainly from reading your posts so now your turn to get dangerous and try a 40W in your auto!! I'll bet you get better figures in your LS1 than 30W.
Hehe, the old switcheroo! [Smile] I am going to be trying a mixture of Schaeffer oil which will put my viscosity in the 12-13cst range, so it will be a high 30/low 40. I do believe with my LT1 engine I will see better results with this viscosity. But I don't think it would need a strong 40wt though, I think if I used an oil in the 15cst area it would start to show lower wear numbers. I may just have to experiment and see though.
 
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I currently have approx 80/20 D1 & M1 0W40. What I'd like to run would be a 50/50 Pennzoil 10W30 Purebase SL/Purebase 5W or 10W50 blend with 3ozs Auto-Rx maint dose. Maybe Johnny could tell me which of the multitude of Pennzoil oils we have here gives me the higher viscosity part? 10W30 Pbase SL I can get everywhere but as there's no Pennzoil Oz web site (why?) I can't work out the other oil. There's stiil some Performax 100 5W50 around but I dont want a dino/syn blend.
 
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I wanted to bring this up again bc it seems to be a constant issue with certain oils. Based on the responses thus far, it doesn't matter if an oil thins out as long as it is still effective. Anyone disagree? If you look at the Corvette, which comes factory filled with M1 5w-30, how many people do you think that extend the drain with this visc. are running a 20wt. oil at some point? And what type of excessive wear could it be causing? If thinning out is not a big issue, I'm inclined to think most M1 analysis is very good. [Smile]
 

Patman

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quote:
Originally posted by buster: I wanted to bring this up again bc it seems to be a constant issue with certain oils. Based on the responses thus far, it doesn't matter if an oil thins out as long as it is still effective. Anyone disagree? If you look at the Corvette, which comes factory filled with M1 5w-30, how many people do you think that extend the drain with this visc. are running a 20wt. oil at some point? And what type of excessive wear could it be causing? If thinning out is not a big issue, I'm inclined to think most M1 analysis is very good. [Smile]
Based on what I see on the various bulletin boards, most new Corvette owners don't come even close to running their 5w30 oil for the maximum 15k. They all still do it every 3k or less. Some even change it every 1000 miles!! (these are the guys that won't drive their car unless the moon and the sun are perfectly aligned) [Smile]
 
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