As long as the oil has a "W" in it, then unless you're subjecting your engine to less than 0 F weather on a regular basis with no engine heater, then the viscosity rating of the oil is neglible for cold starts. The "W" in 5W-30 (or any other viscosity rating) stands for Winter, meaning that the oil is certified by SAE for regular use at temperatures down to 0 F - as long as the oil is API certified, which AMSOIL is not.surgeo1919 said:The thicker the oil the longer it takes to circulate when the engine is cold and that's what wears out the engine especially in the winter.