I have noticed that the viscosity at 100C for many good quality oils which are group 2+ or better is lower for the 10w30 than the 5w. Doesn't seem to hold true for cheap oils. Example: havoline synthetic 10w is 10.1 and 5w is 10.5. Anyone know why a supposidly higher viscosity actually shows thinner? And does this mean that it would be better to substitute the 10.1 10w for a 5w20 than a 5w30? since the vis is lower? Just thought it was interesting.