lubricating plain bearings

Do you mean thick oil which is high viscosity? The choice of oil will depend upon the shaft speed, shaft diameter and bearing material. A good source of info for this may be from folks who are interested in old engines and machinery. You could try the question on the NG uk.rec.engines.stationary. We're brits but not a bad lot for that.
John

I have been using light weight spindle oil in my plain bearing myford ML7. I think they also make medium weight spindle oil.
However I would be very tempted to use 20 or 30 weight non-detergent oil. I think atlas speced 20 weight non detergent oil in their plain bearing lathes.
chuck

The non-detergent stuff is what you want to use if you've got a sump and oil rings or some other non-total-loss oil system. It probably doesn't matter much if your oil goes in the bearing and leaks out to be wiped up and disposed of. Lubricating oil is a totally different product now than when that lathe was built, all you had back then was straight mineral oil with no performance additions and probably lots of wax, or some type of vegetable oil like castor oil. Products evolve, so do standards. You might see what the old engine crowd use in their oil cups for those old low-RPM oil and gas engines. You might also need new bearings cast(or some adjustments) if they leak that much. Rebabbiting is fairly easy to do and Lindsay has the books on how-to.
Stan