Rob, I think you are confusing the cold and hot numbers. With 15w-40, 15w is the winter (cold) weight, 40 is the hot weight. I have never seen a 40w-anything before. I run the 5w-50 Castrol Syntec and get very little oil loss in my 98 Rodeo. The 5w makes it start easier in the cold weather and the 50 protects the engine better in the extreme heat.

As for some more power from a 3.2l, for a little more money on the air filter you can get an HP boost from a Green or K&N style filter. These style filters have been discussed to death here at 4x4 Wire, so try the search function for more info.