The early models had this happen now and then. My 00 did it once in a while, but never consistently. Later models had a baffle built in, to keep the oil from splashing into the breather.
Still, if it is happening consistently, you may have bad rings and are passing compression into the crankcase....then blowing it out the breather. Check the compression and do a leakdown test.
Desert- 450R
Trail- 400EX
Technical Trail- Rancher 420 and 350
Kids- Two 250EX's and 90
Dual-Sport Bike- XT225
Ride Southwest Deserts in the Cool Months, and High Altitude Mountains in the Hot Months!