The bolts on the cover will give away the 10 bolt 12 bolt rear as stated.
Ratio- The axle tube will have numbers /letters stamped into the passenger side facing the front of the car parallel with the ground.
If you get those number we can run them to tell you the ratio. BUT this will only be good as long as the rear is totally original. The one wheel spin to the number of drive shaft turns will work on a posi BUT on a open NON POSI rear it is much less likely you will end up getting a good return in info as spider gears will through off the turns to the wheel even with the one wheel on the ground deal. YOU can get lucky and jack the car up and if you just turn the drive shaft and do not touch the wheel they will spin one revolution as the posi will BUT you can not have any resistance on the axles like the drums rubbing etc. or your again doomed for a correct out come.
THE BEST way is to remove the cover and either count the teeth or look at the outer ring gear diameter (10 bolt only) it will have the ring and pinion tooth count stamped in - On 12 bolts you will have to look to the pinion gear machined top of the gear and a flash light. you will have to keep turning the pinion till you get a clear shot of the top of the pinion...the tooth count will also be stamped on that. Good luck.
Any good brand name oil will do for the rear diff. If you have posi (limited slip ) you will need the limited slip additive to keep the posi unit from chattering. I would advise NO synthetic fluid as the clutch and cone type units are friction base units and need the minerals to make the clutches efficiently stay locked. synthetic is very slippery and will work against what the posi is trying to do...STAY LOCKED...
If the diff is open or you have a true trac or torsen -gear to gear helical worm gear type posi synthetic is fine. Jim