NY, I too recommend using the diesel fuel for a "thinner". In he old days we would often clean the engines before doing the then repetitive valve jobs. These engines that people ran non-detergent oil would sludge up something terrible. We drained the oil added a fresh filter, diesel fuel and started it. We would let it idle for an hour or so. Almost all of the sludge would be gone (emulsified into the oil). The diesel fuel is just a thin oil with a very hi lubricity rate to lube the injector pumps. Never had a problem
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When doing this add what you can and if there is water in the oil add about 2~3 ounces of dish soap as the soap will draw the water into the oil and make the flushing process easier.
Get what goo you can out then add the diesel fuel for the first flush and cleaning~it does work. I've done dozens of engines being restored in this manner. Just be sure to run in neutral with no load IE in neutral.
Dave Neptune