I have used gear and atf oils , and would stick with gear oil , especially for k5 as gear parts are drying up , if you want a smoother and easier changing gearbox you need to pull apart and polish parts that slide on each other which helps greatly , less friction the better, if you have a grabby clutch which most kx250 have , drill more holes in the inner clutch basket which helps greatly on kx250 ,s .I have done this mods on these models for years and are better for it. doesnt matter what oil you use it will never improve the tranny as much as the mods listed here to do .