L/S Rear End Oil 1968 DeVille

[Golf Caddy]

Hey guys, had to replace the pinion seal and with it, the differential oil.  My manual says "SAE 90 Multi-Purpose lubricant conforming to M1L-L-2105B specifications" ??  Not sure what that means and it makes no mention of additive.  Do today's oils, like the motor oil require an additive and if so, how much, 4 oz. - 6oz.  Any help would be appreciated.

[Coupe Deville]

SAE 90 Multi-Purpose lubricant

Modern day equivilent is: SAE 80w-90

I run that in my 72', no issues.

-Gavin

[The Tassie Devil(le)]

Are you referring to a Limited Slip Diff as far as Oil goes?

If the diff has a Limited Slip, then the choise of oil will have a bearing on the type of Limited Slip materials.   Not sure what the Cadillac used, but Clutch Diffs require a different oil to Cone types.

Bruce.

[Scot Minesinger]

I used the exact same oil Gavin suggested on my no-limited slip

[Golf Caddy]

Thanks guys,  sorry I wasn't clear,  it is limited slip.   What about the additive.

Thanks

[Scot Minesinger]

Limited slip or "controlled differential" does require a special lubricant, hopefully a Cadillac owner of that era will respond with the correct lubricant.

[Golf Caddy]

O.K. now I'm thoroughly confused.  With the rear wheels off the ground,  I turned one forward and the other turned forward.   Simple right. ...... sounds like limited slip to me,  however,  lo and behold I open the trunk and there it is, a sticker on the deck lid reading "This car equipped with Controlled Differential."  Should I still go with the 80x90 oil?   I should have gotten the hint when I couldn't find the G2 stamped on the differential.   Thanks guys. 

[The Tassie Devil(le)]

It will still depend on the construction of the Limited Slip material.   Cone or Clutch.

I don't have a '68 Service Manual, so I cannot advise.

Bruce.

[Golf Caddy]

Thanks Bruce,  not sure how to tell the difference between cone and clutch.  Regardless,  at some point I guess I have to trust manufacturer and according to trunk it's a ontrolled not limited slip. According to the 68 manual,  the SAE 90 is what's called for in a Cadillac "Controlled" differential.

[TJ Hopland]

Other than slight damage to your wallet is there another downside to using an oil with an additive?   

[savemy67]

Hello Tom,

I have a '67 Sedan DeVille with a standard differential.  Per the '67 shop manual, the letter "G" is what identifies a differential as a controlled differential, not the number.  The number identifies the gear ratio.  For '67, a factory air conditioning equipped car with a controlled differential would have the number "1" stamped next to the "G".  A non A/C car with a controlled differential would be stamped "G 2".

You may want to take a second look at your differential to see if you can locate a letter "G" or if your differential is stamped with a number only.  In '67, the stamped ID was located on the front-facing surface of the differential carrier housing, at about the 4 o'clock position when facing the pinion flange.  The stamp may be obscured somewhat by the pinion flange and or layers of grime, so look closely.

The rotation of your wheels, and the trunk sticker indicate a controlled differential.  My '67 manual states that the lubricant for a controlled differential is not the same as the lubricant used in the non-controlled differential (SAE 90 multi-purpose gear oil conforming to MIL-L-2105-B).  In '67, the controlled differential used cone clutches.  I imagine that in a worst case scenario, where you used modern 80-90 weight gear oil without Cadillac's special controlled differential lubricant, your differential would lose its ability to function as a controlled differential.  This may or may not affect the long term reliability of the differential as a whole unit.  I encourage you to do more research on the compatibility of modern differential fluids with Cadillac's controlled differential, and on the availability of Cadilac's special lubricant for its controlled differential.

While I have a '67, I think, the engine change in '68 is more apt to have affected gear ratios in A/C verses non-A/C cars, than the requirement for special lubricant in the controlled differential cars.  Good luck with your research and let us know what you find.

Christopher Winter

[The Tassie Devil(le)]

Years ago I rebuilt a 1968 Corvette Limited Slip Diff and used LSD oil recommended by a supposed shop, and everything was going okay till one day whilst taking it for a test drive, and really hitting the gas pedal, the back of the car stepped out sideways, which it wasn't supposed to do.

After many attempts at searching for a solution, the diff was stripped down, and the clutches soaked clean, then a better class of LSD Oil put back, and problem solved.

The gist of the matter is that Clutch-type diffs need a lubricant to allow the plates to slip without chattering, and the Cone type, need a lubricant that is more "grippier".

The Cadillac might not have the power to make the car step out, but internal damage can be caused by using the wrong slippery stuff.

Bruce.

PS.   You cannot go wrong with using what the Factory says.   Sounds like you need to use a single viscosity oil.

[Gene Beaird]

Hmm, according to my 68 Service Manual, 90w is specified for the NON-controlled differential, and for the Controlled differential, you use "Special rear axle lubricant available from servicing Parts Warehouse (standard or Controlled Differential)".  I can find no other specification. 

There is the caution that using gear oil specified for limited slip differentials can cause the differential to chatter in turns in section 4 of the manual.