Anyone know which manual transmission fluid should I use? Should I use synthetic or non-synthetic gear lubricant? I know mobile 1 synthetic oil 75w/90 is good, but not sure it is good for 6th Generation Celica?
I heard that if use mobile 1 synthetic oil for WRX, you will have problem shifting. How about Celica?

75w/90 sounds right, I wouldn't go for anything thinner
it could damage your transmission. I had notchy changes
on mine but went the route of adding a gearbox oil additive
and I gotta say shifting has improved. One thing that filler/drain plug can be a bast**d to remove if it hasn't been removed in a while