hey guys! I have an oil question ---- I thought I understood the oil numbering nomenclature; for instance 10w-30 vs 5w-40 -- the first number 10 followed by W is the cold flow viscosity (5W flows easier than 10W when the engine is cold). the second number is the viscosity once the oil is warmed up (around 200 deg F or so). 30 would be thinner than 40. so here's my confusion -- why the heck does the owners manual say that 5W-40 is only for cold environments and 10W-30 is for warmer environments? shouldn't the 5W-40 full synthetic be the absolute best in both warm and cold environs? I live in MD, where the winter nights can be 10 deg F. and in the summer we get near 100 deg F during the hot days. when my first oil change is due, I was initially gonna put in the Rotella 5W-40 T6, but after reading the manual now I'm not so sure. I can't understand why they would say that in the manual??