Ah, oil… perhaps one of the most heated topics in the motorcycle forums.
My main oil for my KZ750 is Shell Rotella T big jug (though I have been known to thrown in a jug of GTX once in a while). I use the 15w-40, and as I live in the northern mid-west of the US, I ride only at 32F and above. If I ever start riding in colder weather, I might switch to the 5w-40 synthetic. Warmer areas can consider 20w50. Straight-weight oils generate so much debate that I have decided to avoid them altogether, given the availability of alternatives.
I change my bike’s oil at 1500-2000 mile intervals! Bikes that share the transmission with the clutch and the… well, the rest of the engine, break down viscosity quickly. Since it’s only around $13 per gallon, everyone can afford it…
(Update, 2012: Of course, oil is more expensive now… Rotella is around $20 per gallon now)
But one of the most important points of all is this: avoid oil with so-called “enhancements” or “friction modifiers”. These ingredients can make you clutch slip, and you’ll be wondering why your bike won’t move…
Shell Rotella T