Oils have additives in addition to a base oil. As the oil is used, the oil itself stays healthy, but the additive package begins to delete itself and starts to fail to provide benefits like engine cleaning and particle suspension (which carries dirt to the oil filter). In addition, gasoline or diesel gets past the rings as the vehicle is run, diluting and thinning the oil reducing its lubricity and ability to withstand the required pressures required for bearings and cams.
New oil replenishes the additive package, and replaced the fuel-thinned oil.
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