Ahh yes, the almighty “Which oil should I use” topic… Go on any automotive forum and this will be the most common thread topic started. Why? Because there are a lot newbies to these motorized vehicular devices that don’t know much about them…
To make this short and simple, whether your dirt bike is a two or four-stroke, look at what your manual says. If it says to use 10W-40 oil for the transmission, then use 10W-40 oil.
The brand of oil is not so much as important as what kind of oil you use in your dirt bike. In KTM manuals for some of their two-strokes they say to use 15W-50 Motorex oil. If you can afford it, buy the Motorex and you can’t go wrong. If you don’t like spending that much on oil, you’ll be just as well off if you buy a cheaper name-brand 15W-50 oil from Wal-Mart.
Most four-strokes require 10W to 20W oil. W stands for winter, so the first number is the weight of the oil when cold. The higher the number the higher the weight. A lower weight is used when riding in colder weather (40 degrees Fahrenheit or lower), and a higher weight is used for warmer riding conditions in the summer.
Some people have said that certain oils in their four-stroke motocross bike cause the clutch to slip. If that’s the case, go back to using what the manual recommends.
Most two-strokes aren’t very picky with what oil is in them, as long as there is some. Some 5W-30 Mobil 1 from the local auto store will work just fine. Just remember to do your regular oil changes and the engine and transmission will last a long time. A common mistake is letting the same oil be used in the bike, and eventually it will dry up. You won’t be too happy with a seized engine, so spend the few bucks on oil changes.
Just remember that changing the oil regularly is more important than what oil to use in your dirt bike. Filtering out dirty oil is preventative maintenance in itself, and will make your engine last a lot longer.