Differential gear lube in older cars

[JLR]

What was the grade of gear lube used in the differentials of cars in the mid 60's? What would be the best modern replacement?

 

[MetalSlug]

80w90

 

[expat]

What car?
Some may require a GL4

 

[JLR]

65 mustang and a 71 cutlass

 

[aquariuscsm]

https://www.amsoil.com/storefront/agl.aspx

 

[mechtech2]

Normal lube was 80-90 GL5 gear oil . This was necessary for the hypoid ring and pinion gears.
If you had a posi in it, then an additive was often used.

 

[Kestas]

The specifed gear lube for a 71 Cutlass is SAE 80 or SAE 90 GL5 Gear Lubricant. This is straight from the original owner's manual.

The best modern replacement would be any of the following:
SAE 85W-90
SAE 80W-90
SAE 75W-90 synthetic

 

[Bror Jace]

Plenty of gear oils are dual rated (GL-4 & GL-5) ... Chevron 80W-90, for instance.

 

[MolaKule]

In my 62 and '67 Chev Impalas we always used Pennzoil 80W90.

 

[caprice_2nv]

Would there be any harm in using a 75w140 synthetic as a replacement in a car that gets driven hard at times (burnouts, 1/4 mile drag racing, etc.)?

Mine is an 84 Cutlass which originally called for 80w90 as well I believe and I'm ready to change the rear fluid. It has the 7.5" rear end which is definitely capable of grenading after a number of years of hard use. I'm wondering which fluid would make that least likely to happen to me again. If not 75w140 I will just use synthetic 75w90 which I assume would give better fuel mileage on the highway.

 

[mechtech2]

the 75-140 would probably be better, all around, since you drive it hard here and there.
Add that to years of use/abuse, and it makes sense to use it now.
I don't think 75-140 was an option back in the day simply because it was not available.