In general, recomended engine oil viscosity varies on external temperature and mechanical stress put on the engine (like towing). Newer cars have smaller oil passages and tighter tolerances, and have switched to lighter oils. Some are not "synthetic only" or you void the warrantee.

I use Wal-Mart 10w40 in the Amigo, 177k miles.
WHen I switch to 10w30, I burn more oil.
Works fine, even in Dallas winters...though it's a little sluggish in the winters cranking over. But that's mostly due to the battery.

[color:"white"]? 04 Rodeo DI ?[/color] 75k mi, body damage on the 1st weekend I got it.