Transmission and differential oil on 35’ Olds?

[TimFX]

A mechanic told me that I can use the same 80/90 gear oil in both trans and diff. Does this sound right?

 

Its going into a 1935 Oldsmobile

 

 

[Stude Light]

90 wt is fine but I believe that differential uses an offset spiral bevel gear which is called a hypoid. That requires an oil specifically designed for the high contact sliding forces. Not knowing how much brass, if any, is in that rear axle, I suggest a 90 wt GL-4 rated oil. 
 

For the transmission, I run 90 wt GL-1 in my 1939 LaSalle which has some bronze bushings in it. You can also run GL-4 in the trans if you want but the contact stresses don’t require it.

 

Note: Although it is a bit better for highly loaded gears, GL-5 rated lubes have some additives that will oxidize brass and bronze components so they shouldn’t be run in vintage cars that often use these materials.

Edited March 30, 2023 by Stude Light (see edit history)

[Grimy]

On 3/30/2023 at 10:53 AM, Stude Light said:

90 wt is fine but I believe that differential uses an offset spiral bevel gear which is called a hypoid. That requires an oil specifically designed for the high contact sliding forces. Not knowing how much brass, if any, is in that rear axle, I suggest a 90 wt GL-4 rated oil. 
 

For the transmission, I run 90 wt GL-1 in my 1939 LaSalle which has some bronze bushings in it. You can also run GL-4 in the trans if you want but the contact stresses don’t require it.

 

Note: Although it is a bit better for highly loaded gears, GL-5 rated lubes have some additives that will oxidize brass and bronze components so they shouldn’t be run in vintage cars that often use these materials.

Let me add that I recommend you do NOT use GL-6 (synthetic).  I even spoke with a Ph.D. chemist at oil manufacturer S...L... 25 years ago who assured me that nothing in their synthetic gear lube would affect yellow-metal components in my 1936 hypoid diff.  I thought syn would keep temps lower for a cross-country run.  When I went to refill a leaky rear shock at a rest stop in Wyoming, the diff was as hot as the hinges of hell.  I found the nearest auto parts store, purchased GL-4 and a drain pan, and changed the fluid, finding the dreaded "golden sparkles" in the draining syn fluid.  Thankfully, I apparently caught it in time.  Changed again, also GL-4, while staying with friends in Des Moines and found virtually no more "golden sparkles."

Edited March 31, 2023 by Grimy
fix typo (see edit history)