Some of this is pretty funny. I run STRAIGHT 30W in mine and when engine pulled down once @ 100K cam journals were perfect, polished only. The factory low vis oil spec is simply to save a puny amount of gas mileage by friction decrease. I'd rather have the thicker oil, single vis has (or did have) more zinc for anti-wear, thinking that may not be true anymore since EPA demanded reduction in zinc because of phosphate emissions. Single vis oil is actually the stated weight, but multi vis always starts as the lower number weight, then additives make it act like heavier weight. The additives are what break down over time. I change oil/filter at 6000 miles, and my cars last FOREVER. I'm not fond of the idea of running 5W oil in my expensive car. What keeps bearings from dying is PRESSURE, both film thickness and output pressure themselves affect that. The oil pump doesn't give a crap, it's a positive displacement pump and will pump exactly the same amount heavy or light weight. The bypass may go off a little bit different but that's about it. The added friction from heavier weight DOES increase temperature, but I dare someone to do an oil temp test and prove that the difference between two different weights is more than a NEGLEGIBLE amount of temp. That alone will NEVER hurt a motor. Single vis used to be cheaper oil too, just like diesel fuel, you see where that's gone. Have heard of some early zetecs sticking valves from owner using heavier than spec oil, I think Ford just made guides a little too tight and blamed owners later when problems showed up. I have never personally experienced that problem. Don't know about the six, but zetec cam journal clearances range from .001-.003", 30W oil has been used in that clearance range with great success since at least the 1930's.