For a Class 8 diesel engine, engine oil can last 12 months only if mileage stays below the manufacturer's drain interval, typically 35,000 to 60,000 miles depending on engine and oil grade. Additive packages in modern synthetic blends degrade slowly with time. Viscosity holds well. A truck parked for most of the year, accumulating 20,000 miles or less, can run safely on a single oil charge for that period. A truck that ran 100,000 miles in 12 months is overdue at month 8 or 9. The pattern matters for owner-operators with seasonal businesses, fleet trucks sitting at the yard between contracts, or a Detroit DD15 idling overnight at a truck stop where idle hours accumulate without odometer miles. Pull a sample, send it to a lab, and let the spectrometer answer the question your gut wants to.
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