Originally Posted by 67mustang302

The only reason they've gone to tighter clearance is to run thinner oils, because it takes less hp to run an oil pump that's moving solvent(that's what that w20 crap is like). It helps them squeak by here and there to meet the CAFE standards. When internal clearances allow for it, heavier oils are prefered because they provide more protection.

And on this note, this bares mentioning. Viscosity is NOT how thick the oil is. Technically, viscosity is the fluid's resistance to molecular shearing within a given temperature range. It just so happens that viscosity is very closely related to how the oil pours at certain temperatures, so that's how it's measured usually. There is sophisticated lab equipment that actually can measure shear, but it's extremely expensive so the pour method is what is typically employed for rating purposes. That's one reason why(for those of us old enough) if you look at oils today and think back to oils of yesteryear....even a 20-50 oil of today is like water compared to a 20-50 of 10 or 15 years ago. Lubricant engineers have managed to get the oils to be "thinner" and thus flow more easily, while still maintaining the same amount of shear resistance(true viscosity). That's also why synthetics of the same weight are typically a bit thinner and will leak more easily, the oil itself is thinner, but just as/more resistant to shearing forces. That's also why some companies(like Torco for instance) the 10-30 and 10-40 have nearly the same pour and flow characteristics and flow pretty much the same at temperature, but the 40 has a higher shear resistance and offers more protection for roughly the same amount of energy to run the pump. Ultimately oil weight is a measure of the oil's ability to maintain a protective film in the high heat/pressure/friction of an engine. The ideal engine oil would provide the protection strength of a 60 or 70 weight but with the flow and pumpability of a 10 or 15 weight. That's also why there's so much money locked up in good synthetic oils, it's all about providing a lubricant that will rob as little power as possible but provide the best protection at the same time. And there's a tremendous amount of technology involved, much of it in the additive packages.