First off you need to Figure out what it is if you want to decode the 2 letters on the rear of the drivers axle tube by the brake line clip. Firebirds used specific 8.2 Diffs for 7 8 and 9 And you can make them all fit..7 used single leaf but can be converted so the best way to tell is there will be 2 cast in hoops on either side of the top of the carrier housing. 8 does not have these and has multi leaf spring perches. 9 looks identical to 8 except you will find 2 dents that look like they were made with a sledge hammer on top of both axle tubes just to the inside of the backing plates to give more room for the snubbers. But you could also have a Chevy 12 bolt or 8.5 or who knows. the rear cover shape will tell you everything you need to know. If you do have a 8 or 9 8.2 and it has a gear ratio 3,3somthing or higher stock then you hit first gen gold with a heavy duty 4 pinion Safety track with 4 spider/Pinion gears, 2 side gears and a nodular housing. As far as fluid it takes regular hypoid oil with a separate additive for the cones in the locking part. if you cant find the axle code if it is an 8.2 you can use the count the turns method or just pull the cover and if oem gears the numbers for the ring and pinion will be stamped into the side of the ring gear and then you just do the math.