The reason that I wouldn't leave my engine without oil is that there are major temp fluctuations in our Northern climate, causing condensation to develop everywhere. I change my oil and filter at the end of the season when I'm sure I'm not going to ride for a few months. You don't have to change the oil in spring. Just fire up the bike and ride, and whatever condensation that has accumulated will burn off. Bottom line, you don't want your aliminium bits in the engine to oxidize.