Manual Transmission fluid

[JimT]

Thanks guys. I was thinking of trying 75/90 but didn't want to go too heavy. I'm no expert on gear lube and the different weights but I do remember reading that the trans needed a lighter weight than the rears. The rears are currently using either 75/90 or 80/90, whatever TA used years ago. 80/90 was what i intended to refill them with.

What's the difference between 75/90 and 80/90? Is it significant? For some reason I had it in my head that 75/90 is synthetic and 80/90 is conventional and otherwise they are interchangeable. But my lack of knowledge here is why I ask first.