Shell Rotella 15w40 - actually rated for diesels. Sold in gallon jugs at walmart or schucks. Delo 15w40 used to be my favorite in four strokes until they reformulated and added moly to it (bad for wet clutch).
Rotella has a great additive package, excellent base stock. It is relatively inexpensive and can be changed often without hitting the pocketbook too hard. Check out bobistheoilguy.com for more info - they have a motorcycle section there.
You will want to avoid automobile oils with the energy efficiency emblem on them - they have moly additive and can lead to clutch slippage.
