Gear oil and plain bearings is no bueno. If you have a clutch bushing not roller bearing it will eat it. I actually got a Sachs a couple years ago that had gear oil in it and fucked it up bad. Basically textbook of what I learned in engineering school.
I had it in an e50 once and it just felt like it would never fully lock up. The engagement curve would come in similar to 15w40 but at the top it would slip.
Plus it smells like cat piss.
Also gear oil is measured different so 85w90 isn't much thicker than 15w40.