Well, all of my cars spec Xw-30 from the factory (A chevy truck, an 03 toyota, and the miata) so in the summer I like to run the 5w-40 rotella, or the 0w-30 GC. Im actually waiting for it to heat up real nice and I'm going to run 5w-40 rotella + 300ml of Lubro moly motor oil saver in the miata. This will effectively be like running a High mileage synthetic Rotella 5w-45.
Im changing the oil in the toyota tomorrow, Pennzoil HM 5w-30 + a M1 filter.
With that said, I just started dealing for AMSoil, so I might run some AMSoil signature series 5w-30 in the truck for its next change.