[Ernie Vitucci]

Good Morning all...Jim is correct. The original owners manual...which was written by folks in Detroit where it is cold in the winter and warm/hot in the summer, calls for 20wt in the winter and 40wt in the summer. That's what the design engineers expected folks to use. Possible modification if you have an old loose engine. I live in Scottsdale, Arizona and the fellow who built my engine (John Cook) and many others around here believes that 30wt works best for how he built the engine in our weather. Checking with your engine builder is best if you know who did the work. Ernie in Arizona