OK, this is just my two cents....I realize everyone has their own opinion concerning what oil to use, regular or synthetic, and the like.....personally, I have used Amsoil 20-50 for the past couple of years, and have been really happy with it. True, it is a little pricey at $11/qt, (you can get it a little cheaper through a nearby independent dealer), but again, that's just me. As with any oil, you'll find the right fit for you. Good luck!
Duh, I forgot to answer the main question....using Amsoil, I change all the oil (engine, primary and tranny) at the beginning of riding season. On a 1-10 scale, 10 being riding constantly, I would give myself a 5, due to the fact I drive a semi through the week.