I just got a mcss posi rear 3.73 gear unfortunatly I'm not sure of the year and I can't find the ID codes.

I drained the diff oil but I'm not sure how much to put back in?

Which diff oil should I use? Any suggestions would be very helpful.

Are 7.5 rear gaskets all the same?

GM's 7.5" 10 bolts are different across the GM line but Chevrolet is Chevrolet. The differences come in with BOP or Buick Olds and Pontiac.

The axel codes are stamped on the driver side tube at or close to the top. Probably a 3.08:1 my guess, but that is just a guess. The 3.08:1 was a performance rear ratio for the 80s Monty and most other GM cars. The owner could have ordered something else possibly I am not sure of all the options.

Posi Rear ends require an additive that is like Transmission fluid its a thinner oil than conventional 75W90 / 75W140. Its a lot like transmission fluid it protects the clutches and plates in the posi units. Fill until the fluid starts to run out of the filler hole. Cap it, it is full.

You probably would have done better to get the whole rear end from the car not just the carrier. Its probably a stupid question but did you take the Pinion and Ring Gear also or were you just wanting the carrier (Posi)?

Even if your truck had the same Ratio gears the differences in the open face and limited slips can be enough to create a problem when using your pinion with some other ring and carrier. It should all be repositioned and new crush sleeve, shims for alignment. But you did that probably.

Its called Posi Fluid or Limited Slip additive GM Part Number - 1052358.