i have been running 10W-30 motor oil in my tranny for almost a year. the manual also states that is what should be used.
i was wondering what's the best weight to use and if synthetic oil is ok to use. i checked with my local yamaha shop and they lady behind the desk said synthetic may be too "slippery".
then i asked to talk with the mech there and she said he's too busy. so i have turned here for advice.
i have a qt. of mobil 1 synthetic 5W-30 but don't know if this would screw up the tranny?

 

i just use 10 w40 honda gn4 in mine works good

 

Nothings going to screw up the tranny, but it may make the clutch slip, drag, or be hard on bearings. Id stick with what the book recomended. I use motul synthetic in mine, and have never had a problem.

 

synthetic wont hurt a thing, just make sure you use the right kinda, make sure it has NO friction modifiers in it, they will make ur clutch slip, i personally run AMSoil, but from what i`ve heard on here mobile-1 with a red cap is the kind that has no fricton modifeirs in it, and 5-30 should not hurt it, the 5w-30 refers the rateing of the oils viscosity, and the 5W is winter and it refers to how fast the oil drains out of a certain size hole at a certain temp.