Non detergent oils are essential in engines without engine oil filters, such as the early Side Valve Fords, and I know these oils aren't easily found these days.
Detergent oils have lots of additives which are designed to pick up sludge, carbonised grit, etc, and hold it in suspension until it reaches the Oil Filter.
I would not use a non detergent oil in a diesel engine as the greater carbon deposits would fall out of suspension and build in the sump.
They recommend for oil bath air cleaners the same viscosity oil as in the engine crankcase.. but non-detergent, so dust isn't held in suspension and will fall back down into the pan at the bottom, where it settles out and forms a layer of crud in the oil bowl.
Using a detergent oil will hold onto the dust and not allow a layer to form in the bottom bowl. I've always used standard engine oil but having seen this I was just considering a trial using Veg oil.
