Heavier
oil resist the tendecy to flow. Pour a glass of rubbing alchol and compare to a glass of 50wt motor
oil. The advantage of the lighter
oil is it gets into tighter places, (between roller bearings) much much easier than thicker oils. It also flows easier to remove heat and get it back to the oil sump. As far as your clearance you were well with the maximuim of .004-.005". I imagine the signs of premature wear were due to a dirty engine assembly or varnish. You are definelty on the right track with the 180 f thermostat. the problem with the 160F is the pistons get hot, but the cylinders stay cool and you wear your cylinders out. There is a nice chart on the performance unlimited website that clear demonstrates. I though you were running straigth 50w, 20W50 is digestible but still to heavy for most applications, 20w50 is closer to 30w straigt oil. Needless to say you want to minimize the viscosity range since the viscosity modifiers take up volume in the oil but provide no lubricating properties.