Any suggestions on what kind to put in there? i dont want anything special just something i could get at kragens. What thickness is best for it? thanks

 

There is a FAQ "sticky" affixed to the top of the 2Gen forum threads that has a link to free online factory service manuals. The manual for your car will show the correct data for your question, as well as many of your future questions.
https://www.rx7club.com/2nd-generation-specific-1986-1992-17/frequently-asked-questions-2nd-generation-rx-7-faq-fc-494667/

If you are drifting or racing then I would recommend synthetic oil in the differential. Petroleum-based oil is fine if you only drive on the street, although synthetic can reduce chatter and works better in extreme temperatures. I haven't noticed much difference in the differential, but synthetic oil in the manual transmission makes a difference that I can actually feel.

 

Since it doesn't need to be changed very often, cost is not a huge issue and I'd go synthetic even on street applications. The FAQ has a list of good ones. Out of the good ones Kragens might carry Royal Purple, Redline or at the very least Mobil 1. On the street even Mobil 1 is superb. Check the FAQ for weight, though IIRC often they're all the same weight or just say "gear oil" on the bottle without telling you a weight. If the store doesn't have any of the brands mentioned then shoot for one that's a GL-5 and don't worry about getting synthetic. GL-5 is a bad idea for a tranny though, but that's another topic.