Changing rear diff oil on my 68 Deville. Any tips? Other things I should do?

[Gene Beaird]

Roll into the Cadillac Dealer parts department with your FSM in hand and point to the quote and ask the parts guy.  Then carefully watch the look on his face.   

IIRC, for a Limited Slip diff, you'll just need some GM (you can probably also use Ford) Limited Slip Additive in your non-synthetic gear oil of the appropriate weight per the FSM.  You can change the oil and see if you get noise without the additive (it'll chatter when turning tight turns).  If you have the noise, add some additive to see if it goes away.  The additive just gives the gear oil a little more 'slip' cutting down on noise coming from the diff. 

Synthetic gear oils can do that without additives, but 1. some tend to be a little too 'slippery', keeping the limited slip carrier from doing it's work, and 2. could 'clean things up inside' causing more problems than it's worth.  I'd keep the 'dino' oil in it since that's what it's been riding on for all these years. 

[DeVille68]

Roll into the Cadillac Dealer parts department with your FSM in hand and point to the quote and ask the parts guy.  Then carefully watch the look on his face.   

That would be fun! :-)

Synthetic gear oils can do that without additives, but 1. some tend to be a little too 'slippery', keeping the limited slip carrier from doing it's work, and 2. could 'clean things up inside' causing more problems than it's worth.  I'd keep the 'dino' oil in it since that's what it's been riding on for all these years.

Good point! I will stick to the non-synthetic stuff.

From the service manual:

Either SAE 90 multi-purpose type gear lubricant conforming to MIL-L-2105-B specifications or the special lubricant provided for this purpose by the Parts Department can be used for cars equipped with the standard differential.

and:

Cars equipped with the Controlled Differential should use only the special lubricant provided by the Cadillac Parts Department to assure the satisfactory operation of this unit.

So it would make sense that the "special" cadillac oil would also conform to MIL-L-2105-B specifications.

I found an explanation within the book: Lubrication Fundamentals, Second Edition

U.S. Military Specification MIL-L-2105-C:
This obsolete specification requires performance essentially equivalent to API-GL-5. It covers SAE 75W, SAE 80W-90, and SAR 85W-140 viscosity grades. All grades must pass the gear performance tests. The SAE 75W grades replaces the former MIL-L10324A (sub zero) specification, and the 80W-90 and 85W-140 grades replace the 80, 90, and 140 grades of MIL-L-2105-B.

U.S. Military Specification MIL-L-2105-B
This obsolete specification differed from MIL-L-2105-C in that it covered viscosity grades corresponding to an earlier SAE viscosity classification. Generally, only the SAE 90 grade was fully tested in the qualification program.


U.S. Military Specification MIL-L-2105
This obsolete specification described lubricants essentially equivalent to API-GL-4.

So according to this source the Cadillac oil was probably "better" than API-GL-4 but less demanding than API-GL-5.

So I guess that I could use a API-GL-5 with special additives for noise reduction, as suggested by Gene Beaird.

Best regards,
Nicolas

[Dave Shepherd]

Modern gear oil, and the available clutch/posi type additive is available at a GM dealer and most parts stores.