Why is it that in some vehicle manuals it says you can use 10W-30 from 0F - 100F+ and you can use 5W-30 from -0F - 60F? The owners manual even says DO NOT USE 5W-30 ABOVE 60F. Why can't a 5W-30 be used when its 75F outside? What about using an even lighter wieght like 0W-30? I'm just wondering whats the difference because all three are gonna be 30wt oils when hot. Will a 0W-30 or a 5W-30 not protect as well as a 10W-30 when it gets hot outside?