I have recently bought myself two motorcycles and I am sick of paying outrageous prices for oil changes at the shop and have a few questions before I get started DIY. First bike is a 1980 Kawasaki KZ 1000 ltd which is air cooled. The clymer manual states proper oil weight anywhere from 10w40 to 20w50. (currently being run on over priced 20w50 motorcycle oil) What are the pros and cons of using each weight? I live in Ohio so I'm not dealing with any wicked high temperatures. Right now I'm looking at Delvac 15w40, but a little voice in my head says that might be a bit too thin. Second bike is a 1984 honda magna 700cc liquid cooled. Sticker on bike says to use 10w40, but was wondering if I could use 15w40 delvac without hurting her. She has 45k on her already. Appreciate in advance all help and insight.