The Stovebolt Site Forums The Shop Area Engine Shop Non detergent vs detergent 30 wt oil

I think the "no non-detergent oil in old engines" story got started about the time detergent oils were introduced and has lived on in infamy for 75 or more years.

Engines run only on non-detergent oil had a significant buildup of carbon and other impurities coating all the engine parts. When detergent oil was put into one of these engines -- that often didn't have an oil filter-- the carbon and sludge would get pumped through the bearings as particles big enough to hurt the bearings as the detergent desolved it.

All that said, I have taken apart engines that were run on detergent oil but without regular oil and filter changes and they can be are quite dirty inside. A good solution is a fine-micron bypass filter such as a hydraulic oil filter off a tractor or excavator. That will clean it up over time. The replacement cartridges for the optional bypass filters used on stovebolt engines are nowhere near fine enough to be effective bypass filters. They once were but the filter companies changed the filter medium in the last few years to they are the same as full-flow filters these days. At least the ones I've researched are (Wix, Fram, Purolator).