There's no right or wrong answer on what oil to use; you've read the posts and seen all the opinions. Opinions are like ************************************s, everyone's got one.
I just finished up the last of the M1 I had laying around and picked up 10 qts of Castrol Syntec 5W40 on sale for $28(!) for 5 qts plus a spin-on filter a few weeks ago from Pep Boys, which I believe you have in Tucson as well.
Walmart Auto Centers around here stock 5 qt (or 5 liter) jugs of Syntec 10W40, and their price was very good (~$28 as I recall), but I wanted 5W40 and they wouldn't order it for me. Also give Autozone a try, all of the stores here in NJ stock it (@$7.25/qt). If they don't, they'll probably be willing to order it in.