Somewhere on the bottom or bottom outside edge of the gearbox is a drain plug. Drain the oil and put the plug back in place. Somewhere on the top of the gearbox casing, including the left and right outer case covers, you will find a dipstick for checking the oil level. Fill via the dipstick hole. Some bikes don't have an actual dipstick but only have a top plug. Normally you will find it near the kick starter lever. When no dipstick is provided, they expect you to drain the old oil and then re-fill with the amount of oil specified for the bike. Use 10w40 motor oil. NEVER use synthetic oils. They can screw up the clutch.

I have a kymco Stryker 125 99 model. Can some one tell me if the engine oil is seperated from the gear box oil or is it combined,I'm having problems changing gear,bike won't select any gear after 2nd ,it's doing my head in. Thanks.