I guess I'm living in Bizzaro world, having switched to 100% 5W40 in both cars :^) I had been using a blend of 1 qt of Havoline synthetic with Havoline or Mobil 10w30 conventional in the cars, switched to Mobil 1 T&S for the synthetic after I started using it as a blend in the truck, and after a disaster with 100% 10W30 Mobil 5000 in the older car I switched to 100% Mobil 1 T&S. I've switched from 3k to 5k oil change intervals, and am on the 2nd and 3rd oil change in both cars.
It's noticeably thicker in sub-freezing weather but runs very well when warmed up. I have one of the factory manuals for the older car, where a 10W40 is referred to when a motor oil is needed for some sort of assembly procedure, so I'm guesing that the engine was originally designed for a 40 weight. A 30 weight works well enough in the newer car but produced heavy consumption in the older car. A 30 weight would probably be needed for a lot colder weather, but for now I'll keep using it in both.
