Correct me if I'm wrong, but maybe just engine design too? (e.g. cars with turbos, cars w/engines where the oil undergoes large changes in temperature from the valvetrain to the sump, etc....) Maybe some other stuff too. Not saying that I thought synthetic didn't have ANY benefits, but in your case (lots of very long trips, relatively moderate OCI, concern over cost), I think a quality conventional or synthetic blend with a good price would be a better match, that's all. Just an opinion though...
