How do you change the oil on an auxiliary engine and how often should it be done?
Tom Wood responds:
Out of curiosity, I got the manual for my Yanmar out to see what they recommend (perhaps I should consult it more often). I was not surprised to see that they use a standard of 150 engine hours for changing the lubricating oil. I was a bit puzzled that the manual recommends 300 hours, or every other oil change, for replacing the oil filter. Being a little overly fussy with my engine, I change both every 100 hours of operation.
There is a "but" here, though. Every 100 to 150 hours of operation is OK if you use the boat a good deal, but the lube oil should be changed every six months, even if the engine isn't used at all. Oil breaks down over time, absorbing moisture and damaging chemicals from the interior parts of the engine. For many sailors, 150 hours on the engine-hour meter equates to about seven years of usage—certainly not a good thing. The best practice is to change the lube oil in the fall before a winter lay-up, and again in mid spring after it has been used a little.
How to change the oil depends a good deal on the engine and its installation. Most marine engines have a drain plug in the pan that is totally inaccessible on most sailboats. Parts suppliers sometimes offer a kit to replace the drain plug with a hose fitting—a hose attached here can be led out into the light of day where its cap can be removed and the oil drained. If you change your oil often, a pump can be attached to this hose to remove the old oil quickly, and some are even reversible so that new oil can be pumped back into the engine without ever picking up an oily rag.
For most sailors who only change their oil twice a year, a little manual or 12-volt pump that sucks the oil out of the dipstick tube is the inexpensive and logical answer. It can be a time-consuming and messy affair if handled improperly, but with a little practice, some paper towels, and a sense of humor, it is not always a totally unpleasant task. Don't forget to run the engine until it is fully warmed up before you change the oil—this mixes the sediment into the oil, makes it less viscous, and offers you the opportunity to torture yourself with really hot oil.
As a side note, I'd recommend that you use the best quality oil you can find and only the filter made for your engine. The marine environment is hard on engines and driving a propeller-driven boat through the water is the equivalent of pushing a fully loaded truck uphill all day. Not all oil is the same and the difference in price between the cheapest and the best is a matter of a few dollars per change on most auxiliaries. Shell's Rotella and Texaco's Ursa come highly recommended for diesels, and at a minimum you want oil that falls into the API Service Classification CD. The engine manufacturers often add little items to their proprietary oil filters that generic replacements leave off, such as pressure valves or back-flow preventers. Using cheap oil and filters may save you two dollars, but can conceivably ruin a $10,000 engine—that doesn't amount to a bargain in my book.