Motocross bikes get a beating at the best of times, whether you bother to use the clutch or not when changing gear is beside the point, your gearbox gets a hammering too. Replacing gearbox oil on your bike is an central part of motocross maintenance.
The easiest way to keep your gearbox fresh is to keep the gearbox oil topped up to the right level and in prime condition eg: replace it on a frequent basis. Consistently completing this part of motocross maintenance will keep all the gearbox components in prime condition so it won't just function correctly, but it will last longer too.
Each bike is a little different, so it's always good to read your manual. Most two strokes these days are using lightweight gear oil; something thicker tends to hamper the function of the clutch. Four strokes tend to use a multipurpose oil which is used for both the gearbox and engine, so this is even more important to keep up, as the mechanical life of your bike is banking on you keeping up with your motocross maintenance!
I see a lot of people suggesting to change the gearbox oil every 2-5 rides, which really means nothing, after all, some of us are doing 3 hour endurance events, while others are doing 10 minute motocross races!
What I would do is select what you think is right for you, if you drop the oil from the gearbox and its looking somewhat rough, change it more frequently, as it should come out looking fairly fresh otherwise your not giving your gearbox the protection it requires.
Motocross Maintenance - Important Points:
Do not over fill when replacing the gearbox oil, this can blow seals all over the place and cost a lot of money. If you don't have an oil level window, then use a specific measuring jug. If you do have a measuring window on your crankcase, then use it, make sure the bike is standing and that you allow the oil 5-10 minutes to calm down.
If you find metal shavings in the gearbox oil the first time you replace it, this might be because it has not been done frequently. Replace the oil again after your next ride; if it still shows signs of metal shavings, it might be best to refer to a mechanic.
