Diesel engine fluids

John Payne explains the critical importance of lubricating oil for your boat's engine, and how and when to change it

- John Payne

Part 1 of our diesel engine fluids review (PBO, November 2025) looked at the engine seawater cooling and freshwater cooling systems. The next critical fluid-based system is the lubricating oil. "A diesel loves his oil, like a sailor loves his rum" was a line out of the movie Captain Ron, and it rings very true. It's also a commonly neglected engine fluid until something goes awry. The lubrication oil system is critical to engine operation, performance and longevity.

The lubrication system

When a friend indicated that he had excessive oil consumption on his Yanmar engine, we began to review the whole question of what is normal. Our first and most immediate check was to run the engine, followed by a quick check over the stern to assess the exhaust smoke colour. While some blue smoke may be noticed at startup, an accurate check requires the engine to reach normal operating temperature and then check again once the rings and other components have expanded and settled down. When I run my own engine alongside, I always put it in gear and run against the mooring lines to load the engine, as no load running is just inviting problems.

What causes high oil consumption? Gaskets on valve rocker covers and crankcase oil sumps expand and contract over the years and eventually deteriorate, creating leaks. Vibration can lead to retaining bolts losing tension, so a quick check is a first step. Leaks can also be further affected by pressurisation of crankcase oil sumps and valve rocker covers. A dirty or clogged breather is a possible contributing factor. There is a product on the market called Liquid Intelligence, which is specifically for resolving gasket leaks.

The standard answer for increased oil consumption is generally worn, broken or sticking piston rings and excessive liner wear or scoring. Worn valve stem seals, out-of-tolerance valve guide clearances, and valve leakage are also contributors.