Used motor oil is a carginogen. It's toxic. It stinks. I do not want it flung on my skin. I do not want it ruining my chaps and gloves.
As mentioned, even though oils made for diesels have greater base additives like Ca and Mg (TBN) to control the acidity or combusted/partially combusted fuel, the used oil may be corrosive to your equipment. This, combined with fine particles of wear metals, will accelerate the damage of your saw. It is a false economy to employ used crankcase lubricant as a bar oil.
Just about every customer I have is savvy enough to get very upset if I were to use their soil and groundwater as a place to dump my used motor oil. Most of my customers are on very expensive properties, some are like nature preserves. It won't take them (or their lawyers) long to figure out that used oil might contain partially burned hydrocarbon carcinogens, cadmium, benzene, zinc, lead, and chlorinated compounds. It only takes a small amount of used motor oil to contaminate large amounts of drinking water or an aquatic system. Doing a soil reclamation project for $20,000+ is a quick way to minimize profits.