Here is a very informative video explaining oil viscosity. As an illustration, it uses a Corvette 6.2 cut-away engine. The video answers the following three questions: 1. What does oil viscosity mean? 2. Do thinner oils improve fuel efficiency. 3. Can thinner oil effectively protect an engine?
As a former engine oil formulator for Chevron, I find this video to be very accurate in the description of viscosity and oil formulations. Not true for many things about oil on the internet. What is telling is that the push for ultimate fuel efficiency is but difficult and nearly meaningless. No one can measure 0.5% FE benefits in the real world. Driving habits and conditions swamp the engine oil's impact on FE. But OEM's are only trying to meet EPA requirements so they don't care about real world FE. You can make a 10% FE difference by just driving carefully vs aggressively. And who would trade that for the fun factor for driving aggressively. In my view, the most important thing about oil is changing it frequently enough to keep sludge and varnish from froming in the engine. Sludge blocks passages and varnish inhibits ring sealing amongst other issues. I showed in severe duty field testing that it is far more beneficial to change oil than to ever worry about what oil is used in the engine.
One thing not discussed too much when it comes to viscosity is the price in windage one pays when one goes any higher in viscosity. The more like honey an oil is, the more of a big massive clump of goo it tries to turn into, wound around the crank or other rotating parts. David F. Merrion of Detroit Diesel showed me a prototype of a novel idea they were looking at to lubricate the crankcase: a fine mist of some sort of (relatively exotic) lubricant sprayed up under the pistons. I like the idea of thick, viscous, gooey oil, for film strength purposes. (The ability of molecules to "hold hands" like army ants do to form a bivouac or stretch across a stream) However, I can tell you, it ain't all roses and chocolates. I had an oil-gobbling old Satellite once that I dumped straight 60 weight Valvoline in to try to reduce oil consumption. Blipping the throttle with the engine in neutral felt like it was getting a sluggish giant to get out of bed, just to try to increase the revs. It was quite substantial, the "throttle lag" from all the extra windage/friction/whatever. Modern engines don't know how good they have it compared to old school engines.
