Originally Posted By: chicagophil
Is it also true that the HTHS is the primary factor in HP / MPG? Or, with Redline, do you get "something for nothing" in that with the 5w30 you can get a 3.8 HTHS ~40wt "protection" with 30wt "performance"?
My application is a 2007 Honda S2000, which specs 10w30 from the factory (car was released in 1999, with no update to the recommended oil since, leading to speculation that Honda was afraid of "lesser" oils shearing down, thus only recommending the most shear-stable 30wt of the era, 10w30.)
I wouldn't go so far as to say it's a primary factor, but it is a minor factor. The higher HTHS of Redline oils does mean that they will cause more frictional drag in the engine when they are being sheared between moving surfaces. It's just the nature of viscosity: the higher it is, the more resistance to motion due to shearing the fluid, but also the thicker the oil film, and the better the protection against metal to metal contact. It's a balance that must be maintained. If someone were to go to extremes to improve fuel economy by using very low viscosity oil, he would reach the point where the film thickness would be low enough that localized metal to metal contact would occur, and engine friction would increase.
An advantage that I think can be had with an oil that has 40-weight HTHS and 30-weight KV is that it will flow more easily in the regions of the engine that do not subject it to high shear. This could help how quickly the oil drains back to the sump from the cylinder head, and make for more stable oil pressure during extreme maneuvers. Another advantage of low KV would be ability of the oil to transfer heat in the oil cooler.