I think the only concern is that a detergent oil will hold water in
suspension. In an engine the water gets heated out, the lathe doesn't get
hot, so you have to run the lathe up to 10,000 RPM to get it hot enough to
keep the oil water-free or, change the oil out more frequently.
I have a large Monarch lathe built in 1944. It has a geared head,a
plain bearing spindle and some sort of oiling pump to lubricate it. I
use the machine about once a week for half an hour or so at a time, I
live in Canada and my shop is unheated. The belt drive setup was
originally setup for a maximum speed of 500 RPM but I changed it so that
it goes up to 1000 RPM because I came across a very similar Monarch
lathe set up this way from the factory.
I noticed it was low on oil and I was in a rush to fix my tractor so I
added 4 litres of 10W30 motor oil that I had on hand to get it to the
proper level. Is this ok or have I done something stupid?
Having been in the middle of restoring an American Tool Works lathe
which had EVERY wick and oil passage packed off with hardened gunk
(might have been older waxy oil, solidifying in the wicks), I'd urge you
to get it out as soon as its convienent to do so and replace it with the
Not the worst thing you could do. 10W30 will be a bit thin when cold
but this is not the most critical application in the world.
I would not run the machine too fast because with motor oil instead of
regular gear oil, you may get foaming, which is messy. 500 or 1000 RPM
sounds like no big deal.
Keep in mind that if it really does take gear oil, 90W gear oil is
about the same as 30W motor oil in terms of consistency.
You never use detergent oil in a lathe, mill, etc. that does not have
circulating oil pump and a filter!
Detergent oil keeps particles in suspension, which means that the particles
are always going to be wearing away metal. Non-detergent oil allows
particles to settle to the bottom, where that don't do any harm and can be
removed by washing out, whenever the oil is changed. Multiweight oil when
cold/cool is always the lighter of the viscosity number. In effect you have
put 10 weight oil into your machine. Then there is the concern of what are
the detergent additives doing. Are they collecting moisture (likely), are
they compatible with your bearings (old type solid bearings if used), with
your seals? Maybe Ok to use in an emergency by it should be changed.