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Originally Posted by vividjazz
I think it comes down to operating temperatures to a certain extent but how can you mandate the same oil spec with operating temperatures varying so much between places like Alaska and New Mexico or the Northern Territory and Tasmania. One size doesn't fit all.
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Exactly! Its absolutely insane the mandate the same oil for all temps. The Environmental Protection Agency (EPA) tests every car sold in the US in a standardized test to determine city and highway fuel consumption. When the EPA tests the car one of the control factors is the oil viscosity. If the manufacturer wants the engine to have 5w20 oil in the crank case during the test (because it gives 1 or 2% better fuel economy than say 5w30) then the EPA tells the manufacturer that they must do everything in their power to get the owners to also use that viscosity, no matter what the temperature. Ford in the US also mandates 5w20 in all its engines even very large V8s.
EPA letter outlining the use of 5w20 oil.
http://www.epa.gov/otaq/cert/dearmfr/ccd0112.pdf