[Old Henry]

In case some don't know the difference between detergent and non-detergent oils:

Detergent Motor Oil

Motor oil detergents were introduced in the 1950s. The concept adjusts for the failings of oil filters by attaching to those particles too small to get caught in filters. Instead the detergent holds the particles in the oil so they don't deposit on engine parts and cause hot spots. These particles are what makes the oil turn darker. These tiny particles do not harm your engine. If the oil is used for too long, however, it gets saturated, can't hold any more particles, and, in short, can't do its job. Then the particles start sticking to the engine interior. Thus, flushing out old oil regularly helps maintain the cleaning process and the benefit of the detergents.

Non-Detergent Motor Oil

Given the emphasis on keeping engines clean, most dealers and mechanics use motor oil with detergent. It can be a challenge to find a place that does oil changes with non-detergent motor oils. Many of the common non-detergent oils are instead used in appliances that run on gas, such as lawnmowers and tractors. Two-stroke oil would be another common non-detergent oil, mainly used in engines in motorcycles and watercraft.

The Dark Oil Myth

Dark oil does not indicate the need for an oil change. The way modern detergent motor oil works is that minute particles of soot are suspended in the oil. These minute particles pose no danger to your engine, but they cause the oil to darken. A non-detergent oil would stay clearer than a detergent oil because all the soot would be left on the internal engine parts and would create sludge. If you never changed your oil, eventually the oil would no longer be able to suspend any more particles in the oil and sludge would form. Fortunately, by following the manufacturer's recommended oil change interval, you are changing your oil long before the oil has become saturated. Remember, a good oil should get dirty as it does it's work cleaning out the engine. The dispersant should stop all the gunk from depositing in the oil pan.

Read more: Detergent Vs. Non-Detergent Motor Oil |
http://www.nordicgroup.us/oil.htm#Detergent

In all of my studies of oils the only logical reason I have ever heard for using non-detergent oil is when breaking in a new engine so that the metal particles being rubbed off of the surfaces during break-in do not suspend in the oil and cause further wear and damage to the engine and can be flushed out with the first oil change after a very short time. Otherwise, all that I understand indicates that detergent oil is best for the reasons given above. Such would seem especially so for an engine without a filter.

I think it is a myth that detergent oil breaks down sludge. Such myth probably comes from the idea that detergent we use in laundry etc. is to break dirt away from the surface it's stuck to. But, I have not read any actual scientific studies that supports such myth. As the above information explains, the detergent additive only causes a suspension of particles already in the oil; nothing I've read says anything about releasing particles (particularly sludge) from the surface of parts it's already stuck to. Sludge is what happens when you don't use detergent oil. Instead of the particles staying suspended in the oil and flushed out when it's drained, the particles stick to surfaces, particularly the oil pan. So, if you don't want sludge, use detergent oil and change it regularly. If you already have sludge, detergent oil is not going to "break it loose" or clean it out.

I had read that running kerosene through the engine can loosen sludge. I tried it. It may have loosened it a bit but there was just no substitute to scraping it off and out. (That was after my engine hadn't run for 31 years.)