If you want to know when to change oil there is no better way to get optimize mileage and time than oil analysis. The condition of the engine and driving habits as well as where you live all figure in to the formula. Send a sample off and find out if you're able to extend oil changes, or if you should be doing it more frequently. oil analysis also will reflect on the actual condition of the engine as well as the tune up. Let you know if head gaskets are seeping, and coolant is getting into the oil. You may be changing oil prematurely, or you may need to change it sooner than you currently are. Oil analysis will let you know for certain which way you need to proceed
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If you want to safely extend oil changes you need to step up oil filtration to a much higher level through an auxiliary by pass oil filter. There is no downside to having clean oil that is filtered to a much higher standard than any OEM filter will clean it. Running extra filtration, and using oil analysis will let you change oil at optimum times. Without some knowledge about your oil condition, you're just guessing, along with everyone else.
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It's your car, and your decision.