Centex, I agree with your reply, except I think that the second part is reversed. Almost all multi weight oils have a base viscosity of the lower number. They use viscosity modifiers to make the oil function with different effective viscosity especially at different temperatures. The viscosity modifiers are long chain molecules which coil up at lower temperatures and the oil functions at its unmodified viscosity. The coiled up molecules bounce around like tiny balls, which have no effect on viscosity. At higher temperatures the long chain molecules unfurl, increasing the effective viscosity. When the long chain molecules are unfurled, they are vulnerable to shear stress, and they gradually get cut into shorter and shorter lengths. The higher number of the viscosity range gradually reduces, and the oil thins at high temperatures. Small air cooled engines seem to have the oil operating at a higher temperature than most liquid cooled engines. Over on the "Bob is the oil guy" forum, someone measured a temperature of 275 degree F on his air cooled lawn tractor oil filter during mid summer use. This kind of temperature can degrade the viscosity improver molecules and shear them quickly, and I think it explains why many small engine manufacturers maintain their straight 30 weight oil requirement for summer use. I use Rotella 15-40 in all my diesel engines, and straight 30 wt in all air cooled gas engines.
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