Looks like use of non-detergent oil.
"When early engines (prior to 1954) were new, oil filters were an accessory item and non-detergent oil was the type of oil used. Original, unrestored engines have most likely been run on non-detergent oil. If you are running an early unrestored engine that is not spotlessly clean internally, it is imperative to continue to run it on non-detergent oil. Otherwise there is the risk of damage to the engine.
Non-detergent oil was used before oil filters became standard equipment. This type of oil would "stick" contaminants to the sidewalls and valleys of the engine to prevent dirty oil from damaging bearing surfaces. Engines that have been run on non-detergent oil for many years will have a thick "sludge" buildup. Sludge will appear to be oil that has turned to gelatin except that it will be very black with contaminants.
Using detergent oil in an engine that had been running non-detergent oil would allow these contaminants to be released to flow through the engine. This could result in serious damage to the rod, main and cam bearings as well as other engine components such as lifters and plugging of oil lines.