Go to Wal Mart for your M1 oil. Cheapest place in town, but remember that M1 is not a "cheap" oil. The 15/50 is probably the best all-round weight. It has a good low temperature flow (down to -49F) as well as excellent high temp characteristics.

Go here for more oil information than you can ever use:

http://www.bobistheoilguy.com/

Sometimes I get the impression these guys are a little obsessed when it comes to their oil, but ask an oil question on any site and you can start an endless thread. Everyone has an opinion, and this one is mine...

230/8