Another vote for Mobil 1 15W50 and changing it often in a 2 stroke (or 4 stroke for that matter) dirt bike that sees extreme offroad use. I go 2-3 rides, depending on the distance and conditions. This works out to about 100 trail miles. For example, I did the enduro this weekend on fresh oil. I'll do 1 more ride on it of 25 to 50 miles, but would switch it to fresh if the next ride were an enduro since I ride the long course.
Another benefit to the 15w50 Mobil 1 is it works great in 4 strokes too. Just one oil is all you need hanging around for both bikes.
The workin' man's alternative to the Mobil 1 15W50, or for those who prefer to stay away from synthetics, is Castrol GTX 20w50. You can't go wrong with it either, and it runs about 1/3 the cost of the Mobil 1. The clutch action and shifting is so good that I'll stick with the Mobil 1.
As you read these responses, take note as to the weight of the oils. Whatever you decide on, stay away from the 5w30 oils of any brand. These are marked "EC" and contain big spikes of metals designed to increase automobile gas mileage but are bad on the wet clutch in your bike.
Right now Mobil1 15w50 is in my '99 KTM 300E, '98 KLR650, '78 BMW, and '94 BMW. It was in the '98 WR400F when I sold it. The KLR recently spent 10 hours under load (me, luggage, lots of ups/downs, sand) in 110+degrees, riding thru the floor of the Copper Canyon in Mexico. The temp needle was in the upper range all day. At times like this it felt good to have confidence in my equipment, oil included.