The crank isn't churning in the oil, with oil at the full mark, not even close. I keep at full and spend a few dollars more to use synthetic oil. Sometimes I end up with the oil 1/8"over full, not a problem either, no mpg hit, no problems of any kind, nor will there be.
The crank does not touch the oil at the full mark, or even somewhat above the full mark. Only lawn mowers etc have such splash systems. Scroll down to nomograph, and then for the engine cross section. Posted several years ago. The crank is well above the full oil mark. The car is often delivered 1/8" overfilled from Japan. No harm, no mpg hit, nothing happens or is wrong. Do more searching yourself if still in disbelief. Prius Palm Mileage Simulator
