As an experiment, keep your old oil in a large flat plastic dish for a few weeks, then go back and check it. Over time, you'll find thick sludge forming in the bottom, even with oil that wasn't all that old when changed. It's just the by-products of running the engine. I'm sure the oil was lubricating fine (no engine problems) but I don't like the idea of sludge forming in the crankcase, if the bike sits for awhile. So, I change every 2500 miles, or three months, whichever comes first. Just another opinion, and my preference.