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Engine oil turning black shortly after an oil change on a high-hour marine diesel engine is a common occurrence and does not necessarily indicate a mechanical failure.1:11 A 1995 Catalina 36 equipped with a Universal M35 engine showing 3,500 hours often exhibits this symptom due to the typical duty cycle of sailboat engines.0:17 Carbon deposits accumulate within the engine over time, and piston ring wear leads to blow-by, which introduces carbon into the oil.1:07 If the engine runs smoothly, produces good power, and emits clean exhaust smoke, the dark oil color is not a cause for concern.0:40 Attempting to address this as a mechanical problem is unnecessary when the engine performance remains stable.Watch Regular maintenance and operating the engine under a healthier duty cycle help manage long-term wear, but oil discoloration remains a standard characteristic of older diesel engines.1:47

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