oil viscosity and mileage

This depends on a lot of things including the type of oil you use. If you use a good synthetic you can go off of the factory oil life indicator. Its not based off mileage but rather off temperature and engine revolutions. You can use 5w30 for your LS based engine as that's what GM recommends. I like castrol full synthetic, or if you want to spend some money you can go amsoil. Alex

http://www.vadengmpp.com/owners-manual/gmc/2003-GMC-Savana.pdf