Originally Posted by jo_mama

this makes sense


If you can keep your oil dry, then the acid in it can do no harm. The acid needs water to be acid. When people talk about additives depleting in oil, the one additive that depletes the fastest is this water absorber. If you can keep your oil dry, then you don’t need that additive and that additve will last pretty much forever, and all the other additives commonly last much longer anyway. All this is based on my experience in Florida, which is a very wet place to operate an engine. You may get different results if you live in a dry place.

Originally Posted by jo_mama

this makes sense

Lots of people have a favorite guess as to the best oil change interval, but nobody really knows unless they have their oil analyzed regularly. Lacking actual lab results, it is best to guess conservatively, and most do.

Recommendations in the manual are very conservative. Your manual might say 7000 miles, but 20,000 might be completely reasonable for your car and its duty cycle, if you had the lab reports to back that up.

I've studied thousands of reports for hundreds of vehicles. The data covered two and a half decades. Engines are hotter today, but the oil is better. It does not wear out. It becomes contaminated, and that is why we change it. I've seen an engine run four million miles with four oil changes, each due to a failure that put coolant into the oil.

That Detroit series 60 installed in a long haul truck, a Peterbilt, has a very excellent aftermarket filter, which the owner changes every 10,000 miles at a cost of about five dollars plus two gallons of makeup oil, which is sufficient to maintain the additive package. I never change my oil, but I change my filter every 10,000 miles. I use a filter that I know to be better than most because I've also run lab tests on that. My policy is to change cars every quarter million miles, and I've never had an engine wear out before that.

It might be good to add a few words about oil filters. They come in two flavors. One is the less expensive full flow surface type filters commonly supplied by the manufacturer. The entire oil volume flows through it, and its filtering mechanism is similar to a coffee filter. The contaminant particles get caught at the surface, hence the name “surface filter.” From my experience in lab tests, they are very inefficient in catching particles smaller than about 30-50 microns. They also do nothing to take out the water.

The second type of filter is a bulk or "depth" type filter, and it is commonly installed in a bypass or “kidney” loop. It takes a small part of the oil flow, filters it extremely well, and returns it to the sump. Think about flowing your oil through a bale of cotton. Some larger particles will be captured on the surface, but most the smaller ones will be caught somewhere inside of the bale. These can’t handle the great amount of flow that a paper surface filter can handle, as the fluid flows through them more slowly. That is why they are set up in the bypass mode, and they are very effective at pulling out particles down to one or two microns. They can also be designed to pull the water out of the oil through absorption into the filter media.

Bypass filters are becoming very common on industrial and commercial diesels. I bought a number of generators with Cummins diesels in them that advertised an increase in oil change interval from 100 hours to 500 hours. They came with a spin-on filter cartridge with two sections, a full flow and a bypass filter in a single container. By adding a supplemental bypass filter offered by Cummins I could increase the interval to 1000 hours. My lab tests showed that these interval estimates by Cummins were extremely conservative.

I once added the same type of filter as that Peterbilt owner used to a 650 kW generator with a Cat diesel. I pulled an oil sample before adding the filter, and that came back three times dirtier than my threshold for changing oil, but I left the old oil in it anyway. I pulled another sample after running it for one shift, and that sample came back cleaner and dryer than new oil in the drum. The TAN (total acid number) had dropped to nearly zero. Since this engine was twice as big as the one in that Peterbilt, I actually added two of the filters in parallel instead of one.

Perhaps something should be said about water in the oil. One of the more important additives in oil is an emulsifier that keeps water in suspension, such that you cannot see it by visually or physically examining the oil. The oil can hold a great deal of water without you knowing it. But when the oil reaches its limit, the additives can absorb no more and the oil becomes milky. By the time you have enough water in your oil to see it, it is possible that damage has already been done.

If you can keep your oil dry, then the acid in it can do no harm. The acid needs water to be acid. When people talk about additives depleting in oil, the one additive that depletes the fastest is this water absorber. If you can keep your oil dry, then you don’t need that additive and that additve will last pretty much forever, and all the other additives commonly last much longer anyway. All this is based on my experience in Florida, which is a very wet place to operate an engine. You may get different results if you live in a dry place.

The bypass filters keep the oil extremely dry, which is one reason these engines last so much longer. When I said that the Peterbilt owner changed his oil four times in four million miles due to coolant leaks, he found those leaks by oil analysis before they became big enough to be serious. The oil had not yet become milky. The lab reports came back not showing a presence of water, which the filter takes out, but showing instead the presence of elements common to antifreeze and not oil. He can monitor many possible failures by understanding the lab reports from the samples he pulls at every filter change.