[TheFlash]

Synthetic can make a posi slip like crazy.

2011 CTS Diff. OIL SPEC.

SAE 75W-90 Synthetic Axle Lubricant (GM Part No. 89021677,
in Canada 89021678) meeting GM Specification 9986115. With a
complete drain and refill, add 118 ml (4 oz) of Limited-Slip Axle Lubricant
Additive (GM Part No. 1052358, in Canada 992694) where required

 

[Padgett]

Whale oil ?

 

[TheFlash]

No you can't use whale oil anymore that's long gone.

 

[Padgett]

I am already beating the statistics & past my sell by date. May even have a bottle in the garage.