As with a 2 smoke, learning how to change oil on a 4 stroke dirt bike is a very important part of maintaining your thumper and getting the most out of it. It's actually critical that you change the oil religiously on a four stroke as there are so many more moving parts in the engine that need to be lubricated.
If you slip into the bad habit of neglect you'll find yourself on the sideline unable to ride or race because your 'trusty steed' has shat itself and is about to strip you of all your hard earned cash for an engine rebuild.
You should ideally read the user manual for the exact process for your particular brand/model of bike as there are many different engine designs out there requiring certain procedures. Also, some modern 4 stroke dirt bikes have one oil compartment while others have two - one for the transmission oil and another for the engine oil. It's easy enough to figure out if your bike has one or two by the number of oil filler caps it has.
Here are a few easy steps to steer you in the general direction of how to change oil on a 4 stroke.
I'll be demonstrating using Moosey's RMZ 450.
Tip: Use only high quality oil, this will save you money in the long run although it may cost a bit more up front.
Now, whether your bike has one or two oil compartments the process is pretty much the same. It pays to change the oil in both at the same time, using transmission oil for the gearbox and high quality engine oil (10W-40) for the engine compartment.
For those bikes that have a dip stick attached to the oil cap to measure the oil level - warm the bike up first then unscrew the dip stick. Wipe clean with a cloth then sit it back in. Don't screw it back in. This will give you an accurate reading.
Question: How often should I change the oil / oil filter on my four stroke dirt bike?
Answer: About every 5 - 10 hours depending on how hard you race it. You can reuse the filter for another cycle although it's best to swap it for a new one each time you change the oil.
Tip: The cost of repeatedly changing the oil on a four stroke quickly mounts up and is enough to make you consider selling your mothers jewelery to subsidize it. You can avoid that by purchasing stainless steel, reusable filters which should last the bikes lifetime. The key with these is to clean them thoroughly with contact cleaner each time they're removed during an oil change.
Four strokes can be demanding beasts but you can easily keep on top of your oil changes by writing down the hours at which you last changed it. Combine this with regular air filter cleans and you'll dramatically decrease the chance of a catastrophic engine failure.