Hi.
Ok to the above poster, flash point is usually measured in degrees C. 400 degrees C = 752 degrees F. By the way, I have never seen a car's pistons reach 1200 F. No offense, but that is theoretically impossible given the limits of the aluminum alloy used for pistons nowadays. The piston would simply melt. I do not know of any pistons that can withstand 1200 degrees F. The maximum threshold before failure is usually 700 K, which is about 426 degrees C.
A simple way to think of flashpoint is the point at which is vaporizes from a liquid into an ignitable gaseous phase mixture.
The number goes up with increasing oil viscosity because viscosity is directly proportional to the strength of the bonds in the carbon molecules inside the oil. This is why a higher viscosity oil will always provide greater protection by permitting an oil film to adhere to the lubricated parts at higher temperatures. Additives have an effect, but not as significant as the base stock itself.
That's why I believe the science behind these molecules rather than the marketing that is done; when a manufacturer says a lower viscosity oil will provide protection, what they are really saying is "it will provide enough protection for its intended use." And for the most part, people will never notice the difference. But from a science/engineering perspective all factors must be taken into account. Higher viscosity oils simply provide better film strength/adherence at higher temperatures. Those are simply laws of physics, thermodynamics, and fluid mechanics.
