An easily ridden 450EXC should go 2 or maybe 3k on oil before it needs changing. I usually do it in that range for pure "typical" street riding, low RPMs and just running +5-10 over the speed limits. Sumoto pounding is different. Aggressive dirt is different.
The less you use/abuse the clutch for power transfer the less oil contamination you have. No mystery here, ride any bike with a finger on the clutch to get up and over stuff and launch out of turns and you leave bits of clutch plates in the oil. The CRF250 or WR250R are no different. Clutches are clutches, gears are gears and cams are cams. Oil does the same job in all of them.