mtf
 

ive been to a lot of auto stores and they seem to only have up to 85W gear oil. I know that im supposed to use 90W but will 80W or 85W work well enough?

the weight
 

if it says 80W90 is that 80W to 90W?

Yes.
Usually the parts store like Kragen or Autozone have 90W gear oil. I found 3 companies that have it, you could also ask the guys behind the counter if they have any.
John