Flow is gals/min or other units. Restriction is psid or lbs/sq in. in this situation. Why does an 0w get to the heads faster than 40 on a cold day? I think it must be the oil pump has losses or the oil pump relief opens. So use a new filter every change and lightest oil cold rating possible for fastest, filtered, oil to heads. Look at the cold viscosity numbers. it shouldn't matter which filter, like the lady said.