If it is the correct grade and has the minimum rating requirements there is no reason to replace it. Synthetic oil could also be described as better filtered / refined / processed regular oil and its only drawback in most applications is that it is expensive.
In an application where the oil stays clean, no ring leakage or maybe a natural gas conversion, synthetic oils could last for 15,000+ miles between changes and come closer to cost effective. I doubt you would want the same oil in a marine diesel for that many seasons. Michael Brown Windburn C&C 30-1 Date: Sun, 23 Jul 2017 15:45:08 +0000 From: "Hoyt, Mike" <[email protected]> I had my brother pick up 15w40 for me last week. Changed oil today and when old oil drained realized he bought synthetic 15w40. Have outing today and no time to get regular oil. Plan to drain and change later this week to non synthetic Is this an issue? Yanmar 3gm30f
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