Just for spits and giggles, would you mind listing your six different vehicles? Unless you have diesel vs. gasoline engine requirements, I don't see how there can be such a wide variety of oil types and weights as you stated?

.1 MPG difference, where did you get that stat? What kind of oil vs. what kind of oil for a .1 + or - difference?

 

What oil is reccomended for use in Phx Az?

 

Why not consider continue using that which has gotten this puppy to 305K miles!!!

 

I run 5w-30 royal purple year round and am 2 km short of 410,000km on factory engine I bought the truck off my dad when I left home at 189,000 he ran it on 5w-20 Lucas since new didnt even leave the factory oil in it long enough to get warm. But the funny thing about this is if you look at a 2003 f-150 4x4 owner manual here in north america it says 5w-20 in Ireland an identical trucks owners manual states 10w-30 I dug deeper only north america recommend 5w-20

 

The oil today is a completlely different "animal"....... Toyota as an example in the prius uses 0w-8 and 0w-16......IMHO, look at the mfg specs for atmospheric conditions and the oil mfg recommendations and go from there.