Many new cars now ship with full synthetic and have 10,000 mile intervals. I've never gone anywhere near that on the bike, but since our oil isn't shared with gearbox/primary I wouldn't think it picks up contaminants at a rate that much quicker than a car. I guess if the drain plug is catching a lot of metal, short intervals would be a good idea (but not as good as fixing the issue)...

 

just changed my oil at 5000 , revTec synthetic....primary next, revtec chain oil-5000...and primary at 10000

 

Here is a curve ball for you. S&S cycles say 1k mi dyno and 2k mi synthetic in the air cooled engine.