Detergent motor oils have mostly unknown additives. And machine tools have mostly unknown alloys of brass/bronze used in some of there parts. From my reading it's fairly rare,and it seems more common with the older machine tools. But there are some yellow metal alloys that react to and can be eaten up by those additives. Unfortunately there's no way to tell in advance if you'll have a problem or not until it happens. So it's just safer to not use them at all. Most times any oil is better than no oil, but not always.
For something like sleeve bearings on a lathe spindle, substituting a heavier or lighter weight oil than the factory recommendations may not allow the proper oil wedge to form. Without that you'll have metal to metal contact. The correct bearing clearances are engineered around the correct type and viscosity of that oil recommendation. Anyone who isn't a lubrication scientist or engineer for there day job who thinks they know better than the machine tool designer already doesn't know nearly enough to do so. The correct oil is dirt cheap, having to replace or rebuild high precision parts isn't. Before I learned better I tried just about everything commonly available from 3N1 sewing machine oil, chain saw bar oil to 90 wt gear oil on my machine tool ways. None of that comes close to what the proper Vactra no. 2 can do. I'd also bet money I don't have that more machine tools are worn out in amateur and even professional shops by poor to little cleaning and lubrication habits than are ever worn out actually machining parts.
