— The website of the Sailing Vessel Beatrix, Kelly-Peterson 44 #276 (1980).

Active cruising raises new issues. One of these is carrying lots of lubricating oil, finding places to dispose of it, and the much higher expense of purchasing it. I learned that an oil bypass filter can seriously extend the life of engine oil.


If you can also use synthetic oil this will increase the oil-change interval enormously. And I understand that synthetic oil can be used in older engines. However, because we may be in exotic places, I decided to stay with my 40W normal engine oil in installing a bypass filter. I am still hoping for a several times the life.


Oil Lifetime

Abrasive particles in the oil, not the oil itself, is the issue with oil change interval. The abrasives are the microscopic (2 - 60 microns) result of normal wear during operation. The longer these abrasives remain suspended in the oil the more they will wear the critical very tight tolerances of the diesel engine.


Primary filter

The full-flow filter at 20-30 microns allows the high-volume flow necessary for lubrication. The filters must work without introducing a lot of restriction or else oil will not flow into the engine during cold start-ups. This is one reason the full-flow filter allows passage of the comparatively small contaminants; trying to catch everything would restrict flow. Those abrasives smaller than the filter sieve size will remain suspended in the oil and continue to cause wear on the engine.


Bypass Filter

Secondary filters take a small portion of the normal oil flow, usually less than 10% (sometimes only about 1%) and subject it to additional cleaning. Secondary filters are better known as "bypass" filters, but they act separately from the primary filter and have nothing to do with an engines bypass valves. They extend oil life by taking the burden of filtration off of the primary filter. With a 2-3 micron sieve size, and on its own oil circuit, It filters out the small abrasives which are ignored by the full flow filter. Tip: When the filters are no longer warm while the engine is in use they are clogged and need replacing.

Dual Filter Plan

Each type of filter acts to accomplish different jobs in the overall filtration plan and the oil remains relatively clean for much much longer. The oil has a long life if kept clean and if you do not exceed normal operating temperatures. The diagram below illustrates this plan.


Oil Analysis

I have used oil analysis to establish a baseline for evaluating oil change intervals with the new dual-filter system. Pre-paid oil analysis kits are available. You take a sample and send it in to the lab. The results are returned via email or post.

The first oil analyses for the Perkins 4.154 included two tests: one before the most recent oil change, and the second sample after the oil change.

30 May 2010 (Adobe PDF File, 63.9 KB,

Jun 10 2010 06:48
26 Aug 2010 (Adobe PDF File, 44 KB,
Oct 28 2010 00:36

This gave me a baseline for how long the oil lasts with the new bypass filter system. It looks like, from the first test, the oil was in pretty good condition after 150 hrs. At $100/40L the oil tests may pay for themselves! I think I could extend the oil change to 200 hrs even without the bypass filter system. The next test shows no degradation in the oil. Contaminants and metals content values indicate the goodle Perkins is in excellent condition.

Final test results show that engine oil viscosity is holding up after 400 hours, but soot is climbing.

28 May 2011 Cumulative (Adobe PDF File, 50.6 KB,

Oct 15 2011 19:37

The test also notes high sodium, which is a potential problem with the coolant system, not the oil. The tests indicate that a 300 hour oil change interval is perfectly OK, with a 400 hour oil change not posing any problems.


With the aid of Craigslist I found a very helpful and knowledgable individual, who sold me an older model Amsoil dual-filter bypass kit normally used on much larger engines. The dual filters should mean an even longer life before they become clogged, and all we had to do was change the restriction orifice that limits the amount of oil diverted to the bypass system. Also see the Amsoil web site for more information on synthetic oil and bypass filters.

At this writing, 400 hours after installation, I am noticing it takes longer to heat up the bypass canisters, which means either they are clogging up or the orifice is too small.


25 May 2010, Brooklyn, NSW, Australia


The contents of this page are an example of a refitting job performed on a single vessel by its owner.  Just because I put it here for your interest and information does not guarantee it will work on your boat, or work at all. Fair winds & smooth sailing. -- Jeff Stander


Last modified: February 15 2021 21:37