Differential Oil Dilemma

[ODRotorHead]

In order to establish a baseline for routine service intervals on my new-to-me A2, I decided to change the gear oil in the differentials. The -10 (March 1983) was handy, so I opened it up to check the required specs for differential gear oil.

Table 1-4 on page 1-8 reads; “All - Differentials - 2 pints - GO 85/140.”

Same -10, . . . Appendix D, Section II, Expendable Supplies and Materials List identifies the required oil as “Lubricating Oil, Gear Multipurpose. GO 85/140 (MIL-L-2105C).”

Having confirmed the viscosity requirement, and having done a reasonable amount of research regarding API and SAE specifications (GL numbers, sulfur additives, synthetics, etc.), I settled on Valvoline “High Performance” SAE 85W-140 Gear Oil (not a recommendation - please let me know if that’s a bad choice for the differentials).

Information on the Valvoline label includes, “Designed for use in non-synchronized manual transmissions where GL-4 is specified, and for hypoid differentials where GL-5 is specified.”

Satisfied that I’d done due diligence, I changed the oil in both differentials and took the truck out for a drive. All good, no leaks.

Fast forward a couple of weeks and I find myself looking over the LO from 26 July 1988. Two weeks earlier, I filled the differentials with SAE 85W-140 based on the specification listed in the 1983 version of the -10. Now I discover that the gear oil specification (down to -15F) in the 1988 LO is listed as GO 80/90. Dang.

Going back to the LO dated 28 July 1978 the oil viscosity specification for the the differentials was GO 85/140. Same specification I found in the 1983 -10.

Going back even further though, the -20 dated 23 September 1971, specifies (down to -10F) straight GO 90 (MIL-L-2105).

Anyone have any idea why the viscosity specification flip-flopped from GO 90, to GO 85/140, and then back to GO 80/90???

Is one better than the other in terms of lubrication? More importantly though, is one or the other more likely to cause problems such as leaks or blown seals??? I doubt I’ll ever drive the truck in ambient temperatures below 50 degrees F (no heater and a missing left-rear curtain), but I’m still a little concerned about having 85/140 in the differentials, . . . as opposed to the current spec for 80/90.

Should I change the differential oil again?

Thanks.

[rickf]

Interesting, I have never seen the 85/140 recommendation. But, that is what I run in mine. If you have ever felt how hot they get in normal driving down the road you would understand the change. You will be fine with what you have. The wider the spread in viscosity in a rating means there are more and longer polymer chains in the mix. Get hold of Raymond for the gory details. That was his field of expertise.

[ODRotorHead]

Excerpts from the tables . . .

Transmission Oil Requirements -10 001.jpg

Transmission Oil Requirements 002.jpg

If you haven't had any problems running 85/140, that's good enough for me.

Thanks for the advice Rick.

[rickf]

I know guys running straight 90 and they have done it for years with no problems. For what we do with these vehicles as collectors we will most likely never have oil related issues as long as it is changed on a schedule and kept full. Engine oil is the one you need to watch due to having flat tappet lifters and modern oils not having the sulfur, zinc and phosphorus in it anymore. They have all been mostly removed because they were harmful to catalytic convertors. That is why most people run 15W-40 diesel rated motor oils since they still have more of those additives in them. Another option is to add ZDDP supplement to your regular oil. Be SURE to read the instructions!!!!! Adding to much can do more damage than not having enough. This is one place where more is definitely not better.

[raymond]

Shell used to say you could go one grade heavier on just about any oil (motor, gear, hydraulic, turbine, etc.) but should not go with a thinner oil unless using the equipment in very cold weather conditions.

[rickf]

Raymond, I remember many years ago in a seminar I attended on oils we were told about long chain polymers used in the multi-grade oils and it was better to stay with the closer spreads like 10-30, 10/40 over something like 20-50. Now, this was 40 years ago and I am sure things have changed since then. what do they say nowadays? I use the 85-140 in my mutt due to the extreme high temps in the diff.. For those that do not know the gear ratio in a mutt is 4.88:1. So those gears are really spinning at road speed. Just cruising around town is not the problem but if you are doing 50-55 mph on the highway and then check the temps on that rear you will be amazed.

[raymond]

Not sure what the experts are saying today, as I have been out of it for quite a while.
Don't think I told you this, but we sold the family business a couple of years ago.


That being said, 85/140 is actually pretty close in terms of viscosity when you consider that gear oil and motor oil viscosity is measured with a completely different scale.
I.E. there is not a lot of difference in viscosity between 90 and 140 weight automotive gear oil, especially when compared to similar viscosity motor oils.
90 weight automotive gear oil is roughly in the 50 weight motor oil range while 140 weight automotive gear oil is roughly in the 70 weight motor oil range.

https://www.thelubricantstore.com/under ... rade-chart

[rickf]

I knew you would have some answers. Interesting that I have never seen that chart.