For a Class 8 semi running long-haul, 7 months is a long time if it includes more than about 40,000 to 60,000 miles, and a short time if it includes less than that. Modern synthetic-blend CK-4 and FA-4 diesel oils hold their viscosity and additive package well beyond a calendar quarter, but the actual decision driver is mileage and fuel quality, not months. A truck that sat parked for half the period accumulated no contamination and lost only some additive package over time, which degrades slowly. A truck that ran 80,000 miles in seven months is overdue regardless of season. On a Detroit DD15 or Cummins X15 at standard line-haul intervals, 7 months is borderline normal. On a Mack MP8 in vocational refuse duty, 7 months is well past due. Send a sample to Blackstone or Polaris if uncertain.
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