Well, I hadn't thought about the possibility of holding metallic dust in suspension but that would be another difference between HD engine oil and non-detergent oil. The difference that I see quoted most often involves the detergent designed to scavenge moisture (water) from the oil in the sump until the next time that the engine is run and the elevated operating temperature blows the water out the tailpipe. So that would be two reasons not to use Hi-detergent engine oil in a machine whose operating temperature never gets much above ambient.
One other significant difference between the operating conditions of most lathes or mills and most typical IC engine is that the latter almost always includes a pump and filter to re-circulate the oil and to remove the fines from the oil stream. Whereas most machine tools do not. The majority of lathes and mills only use the oil a few times as it drips slowly through the machine and out onto the oil pan or other surface beneath the machine. Generally, only the larger and usually much more expensive machine have a sump and and pump to recirculate the lube oil.
However, everyone is free to do as they prefer.
