Would 85w_140 gear lube be ok running in the transmission? It says it's for off high way vehicles and manual transmissions.I can get a 2 gallon jug for free from work still sealed but not sure if it's too thick for my 500 or 250.

Your gears would like it but the clutch will hate it.  It is too thick for a wet clutch and will probably make the clutch slip if the motor makes very much power. 

Use a light weight oil like ...10w30 in cold or 10w40 hot weather