Combines oil in cylinder gearbox

80w90 is just fine unless you really want synthetic. Our rule of thumb was to just make sure you change the oil, no matter what kind it is, before each harvest season(once a year). Cheap insurance for a basically $5000 gearbox. hope this helps!

I would just go with the synthetic gear lube. Yes for sure change it every year. No more oil than it holds, the synthetic oil will not cost much extra and i have seen its benefits.

And check the oil level everyday. We all know how quickly a gearbox can burn up if the output seal leaks into threshing grain.