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I have an ICE car which is driven about 2000 miles per year. Use is split between short and long trips in a warm climate. It came with synthetic blend oil but now runs full synthetic. Everything I could find says never go more than a year between oil changes without regard to mileage. However, I have been unable to find any objective data showing how much slipperiness the oil loses simply due to the passage of time. Is such objective information available or is the one-year interval something left over from the days before synthetic oil, a manufacturer's CYA or an old wives' tale?