Fluid change M22 and 373 posi

[1967CorvetteCoupe]

Any advice would be appreciated as to brand and weight oil to use in my '69 Z28. The car has an M22 trans and posi 3:73 rear. Looking to change both.

[camaronut]

I'm sure JMN chimed in on something like that on this forum.  He would know, or just follow what the GM manual recommends, except maybe go synthetic. 
Not sure how that would work mixing synthetic with residual dino-gear oil.

[cook_dw]

Do not use synthetic oils.

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[bcmiller]

Do not use synthetic. I buy this by the gallon but you can get it by the quart.

For the rear end, you will want to put in the GM posi additive too.

[maroman]

What did GM switch to for posi additive since they quit killing sperm whales?

[bcmiller]

This. Not sure exactly what is in it. I sometimes use the Yukon brand additive too.

[maroman]

I just remember the clear bottles of stuff from the '60's that really worked. I knew a parts manager that ordered hundreds of cases when it was first discontinued. That may have been in the early '70's when it was not PC to kill sperm whales..