I have decided to use a 75W-90 GL-5 synthetic gear oil for my upcoming front and rear diff fluid changes on the Burb, but there are quite a few choices out there. Aside from personal preferences and testimonials I have read on BITOG (most of which have been comments that such-and-such brand works fine in the poster's application), I have not come across anything that really explains how to compare fluids in the same grade to determine what might be the best choice in a certain application. I could go the route of doing multiple UOAs using different fluids, but the drain intervals for my application are 100k+ miles (in fact, the owner's manual includes fluid level checks every 7,500 miles, but never calls for fluid changes), so that does not seem to be a very effective way to go.
Is there a simple way to match a gear oil's performance specifications to an application to choose the best fluid? Gear oil PDSs do not contain much information, so can any reasonable conclusions be drawn by comparing viscosity and VI numbers for fluids in the same grade? Are 100C and 40C viscosity numbers even directly applicable to differential operations (I have no idea what normal differential temps are)? How much of a role does VI play?
For my particular application, the Burb is used primarily as a kid hauler, with the most frequent trips being 12 to 25 miles each way, mostly highway. It will also see road trips of 700+ miles once per year (with a 3,000 mile round trip that involves passing through the Rockies coming up in a few months). In the nearly two years we have owned the Burb, we have never used it to tow and I do not see doing so any time soon. I am located in Southern California, so this vehicle is not driven in low temperatures, and if it ever is, it will most likely be above 0F. So I am basically looking for a fluid that provides adequate protection under this light duty cycle and any possible MPG benefits from reduced friction during short to medium distance driving. In my quest to compare the available fluids, I put together the following table based on current PDSs, with fluids listed lightest to heaviest at 100C:
Code:
Fluid 100C, cST 40C, cST Viscosity Index
Mobil 1 LS 14.6 103 146
Castrol Syntrax 15 111 ???
Valvoline SynPower 15.6 100 150
Peak GL-5 15.9 104.7 162
Redline GL-5 16.4 115 155
AMSOIL Long Life 16.6 129.7 137
AMSOIL Severe Gear 16.8 109 167
Royal Purple Max Gear 17.5 119 ???
If lower viscosity equals better fuel efficiency (is that a valid assumption?), looking at it on paper, Mobil 1 appears to be in the lead when it comes to overall fuel efficiency due to it having the lightest viscosity at 100C and second to lightest viscosity at 40C, although it is among the lowest VI. If I was planning on moderat towing or occasional mid-high load, mid-high shock runs, I should lean toward RP or AMSOIL (as I understand it, if I was going heavy load/shock, I would bump up to 75W-140 or 85W-140), right?
So based on the information and assumptions above, product availability and price, and testimonials I have read on BITOG, I have decided to go with Mobil 1. Are there any flaws in my reasoning? I am looking for gear oil education as much as a specific product recommendation here.
[As an aside, I found Valvoline High Performance Gear Oil (conventional) to have interesting numbers of 15.47 cST @ 100C, 99.0 cST @ 40C, and a VI of 166, which, if compared to the fluids in the table above, would be the third lowest 100C number, lowest 40C number, and second highest (by 1 point) VI. Maybe it is a viable option with 25-50k drain intervals?]
Is there a simple way to match a gear oil's performance specifications to an application to choose the best fluid? Gear oil PDSs do not contain much information, so can any reasonable conclusions be drawn by comparing viscosity and VI numbers for fluids in the same grade? Are 100C and 40C viscosity numbers even directly applicable to differential operations (I have no idea what normal differential temps are)? How much of a role does VI play?
For my particular application, the Burb is used primarily as a kid hauler, with the most frequent trips being 12 to 25 miles each way, mostly highway. It will also see road trips of 700+ miles once per year (with a 3,000 mile round trip that involves passing through the Rockies coming up in a few months). In the nearly two years we have owned the Burb, we have never used it to tow and I do not see doing so any time soon. I am located in Southern California, so this vehicle is not driven in low temperatures, and if it ever is, it will most likely be above 0F. So I am basically looking for a fluid that provides adequate protection under this light duty cycle and any possible MPG benefits from reduced friction during short to medium distance driving. In my quest to compare the available fluids, I put together the following table based on current PDSs, with fluids listed lightest to heaviest at 100C:
Code:
Fluid 100C, cST 40C, cST Viscosity Index
Mobil 1 LS 14.6 103 146
Castrol Syntrax 15 111 ???
Valvoline SynPower 15.6 100 150
Peak GL-5 15.9 104.7 162
Redline GL-5 16.4 115 155
AMSOIL Long Life 16.6 129.7 137
AMSOIL Severe Gear 16.8 109 167
Royal Purple Max Gear 17.5 119 ???
If lower viscosity equals better fuel efficiency (is that a valid assumption?), looking at it on paper, Mobil 1 appears to be in the lead when it comes to overall fuel efficiency due to it having the lightest viscosity at 100C and second to lightest viscosity at 40C, although it is among the lowest VI. If I was planning on moderat towing or occasional mid-high load, mid-high shock runs, I should lean toward RP or AMSOIL (as I understand it, if I was going heavy load/shock, I would bump up to 75W-140 or 85W-140), right?
So based on the information and assumptions above, product availability and price, and testimonials I have read on BITOG, I have decided to go with Mobil 1. Are there any flaws in my reasoning? I am looking for gear oil education as much as a specific product recommendation here.
[As an aside, I found Valvoline High Performance Gear Oil (conventional) to have interesting numbers of 15.47 cST @ 100C, 99.0 cST @ 40C, and a VI of 166, which, if compared to the fluids in the table above, would be the third lowest 100C number, lowest 40C number, and second highest (by 1 point) VI. Maybe it is a viable option with 25-50k drain intervals?]