Couple of things affect oil life. (a) how much time the engine is on the choke warming up. You are washing the cylinders and polluting the oil. (b) Riding long enough each time to get the engine FULLY up to temperature (getting the oil good and hot) this "boils" the moisture and volatiles out of the oil. (c) How effective the air filtration system is. The bigger the pores in the filter, the more crap makes it into the motor. (d) How long does the bike sit unridden.
For you guys with fuel injection (A) has little effect. In fact most of the engine wear (aside from inhaled dirt) comes from those first few minutes after the engine is started up cold.
Personally, I change mine every 3K, sooner if it begins to use some. (And some oils "go thru the motor" more than others.) In fact, I run my 250 so hard, I am amazed it hasn't come unglued by now. I truly run the livin' crap out of it. Wide open 13K shifts all day long on my Sunday rides. (tame and easy in town where the PoPo live
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