transmission and differential fluid? (5 speed, non turbo, rx7 fc 1987)
 

what kind of tranny oil and differential fluid do I use for my car?

Regular, good ol' gear oil, 75-w90 does the trick for both. Some of the better shops recommend Redline or Royal Purple, along with GM 's LSD additive if you have a Clutch-type Limited Slip Differential (86-88 only). IIRC, only GXLs and TII's have a LSD.

i like to mix about 50/50 gear oil and atf in the transmission in a street car, it acts like synthetic, but its cheap and easy to find.

Unless you are Pounding the shit out of the car then the regular 75w90 or so will work just fine.
It's cheap and works.
Now if you want you can go Half and half with a Synthetic gear oil and a 75w90(which is really what they refer to as Semi-Synthetic) or Hop up to a full synthetic which in the long run smoothens out the "glitchy gears".
Honestly for a car that won't see the track or racing then just get something that works.Regular Gear oil.
On the rear diff you can add a "modifier" (Royal purple has it) If it is LSD.
If it's a "one wheel Peel" type diff then don't waste the money.

this is something i got hip to in the last few years myself.