Right now I'm using M1 5w30 in the winter and will switch to 10w30 in the summer. Some folks say that M1 5w30 is to thin and to use 10w30 year round. Would this be ok in Colorado which I've seen it get to -20 degrees F in the coldest part of winter? The cars are a Silverado with a 5.3 liter Vortec engine and a Camaro Z28 LS1. Thanks!