The gear oil goes into the gearbox not the crankcase. The oil in the fuel lubricates the piston, cylinder, rings, bearings etc... The filler cap is on the right hand side of the engine, when you sit on the bike, it will be a plastic hand screw bung usually black in colour. When I change the oil on my bike I run it for about 2-3 minutes until the oil gets slightly warm and its viscosity becomes runnier, I then unscrew the filler cap, unscrew the drain bolt on the underside of the gearbox and drain the oil into a container fully. Once drained I tighten the drain plug, and fill the gearbox up with oil, I use 10W 40 but I'm sure there are others you can use. I am not sure about your gearbox oil capacity but my 250 is 0.8L or until the oil reaches the lower part of the window on the right hand side.