Jarhead Jim:
You are correct about our recommendation for the oil. Tractor hydraulic Oil is what is known as a "DTE" oil. DTE is an ancient designation that predates the automobile. DTE = "Dynamo, Turbine, Engine". DTE oils are a straight weight, mineral based oil and come in grades: Light, Medium, Heavy-Medium, Heavy. An ISO 46 grade tractor hydraulic oil would correspond to a DTE Medium and is fine for plain bearings in your LeBlond Lathe. Tractor Hydraulic Oil has anti foam and anti corrosion additives, and these are OK for your lathe bearings.
ISO 46 or ISO 68 are the common weights for tractor hydraulic oils. ISO 46 is roughly equivalent to an SAE 20. SAE = Society of Automotive Engineers. The SAE did not come into being until automobiles were fairly well established and being produced in considerable numbers. Dynamo is an oldtime term for a Direct Current generator, and engine refers to steam engines, while turbine refers to hydroelectric turbines or steam turbines. ISO is the most recent system of oil designation and stands for "International Standards Organization", something that came into being in more recent times as industry became more global. Quite a little bit of history in the development of oil designations. Your LeBlond lathe likely predates most automobile manufacturing and was designed at a time when hand forged carbon steel lathe tools were the most common thing. Working on these old machine tools, it helps to put ones self in the mindset of the era the machine tools were designed and used. Carbon steel cutting tools = low cutting speeds, and oil would be a loose spec, probably something like: "Use a good clean engine or machinery oil in the spindle bearings..," Where 'engine' was likely referring to a steam engine with babbitted bearings, hence a DTE oil.
Gear Oil, the 80-90 wt you mention, is too heavy for the bearings. The bearings may use a felt wick to bring the oil up from reservoirs in the headstock beneath each bearing. These reservoirs are also known as 'oil cellars'. The felt wick utilizes capillary action to cause the oil to 'climb the wick' and wet the spindle journals. A heavy bodied oil like gear oil, is going to take its sweet time in climbing up the wick.
If the lathe seemed to run OK with the gear oil lubricating the headstock spindle bearings, I find myself thinking the clearances on the bearings may be a bit on the loose side. A heavy bodied gear lube and the close clearances a lathe headstock bearings should have seem like a bad combination that would result in the bearings running hot.
Without knowing the size of your lathe and the nominal sizes of the spindle journals, I am taking a rather big WAG to try to give you any kind of bearing clearance values. I would not be surprised if LeBlond did not set the headstock bearing clearances at something like 0.002-0.003" for the smaller bearing, and perhaps 0.003"-0.004" for the larger bearing (closest the chuck). Time and an unknown history of owners and how they maintained (or did not maintain) the lathe all work against the bearings having those tight clearances at this point in time. Opening the bearings by lifting the bearing caps may tell some of the tale when the condition of the spindle journals and the bearings can be seen.
Checking the bearing clearance is easily done with a pry bar and a dial indicator. The method is:
1. slack the drive belt and back off the thrust adjusting nuts on the spindle. Disengage the feed/threading gearing (neutral position of the lever).
2. Set up a dial indicator so it touched the collar of the spindle (between the threaded spindle nose and the bearing cap) at 12:00 and put some pre-load on the indicator. Zero the indicator.
3. Stick a piece of hardwood such as a hammer handle into the spindle bore and push down hard. The indicator may move in the 'minus' direction. What you are doing is squeezing out any oil film between the bearing and spindle journal.
4. Zero the indicator with the spindle in the 'down' position. Visualize the spindle journal touching the bearing at the 6:00 position. In theory, the contact between
two circles of slightly different diameters, one inside the other, is "line contact". In actuality, it will be a large contact area.
5. Using a pry bar (and a piece of copper to protect the spindle threads or machined collar surface) and a piece of wood blocking on the lathe bed as a fulcrum, pry the spindle upwards (prybar contacts the spindle at 6:00).
6. Note the indicator reading. This is the bearing clearance.
The proof is in how the lathe turns a piece of stock held only in the chuck. Without benefit of a tailstock center, the stock held in the chuck is cantilevered out from the chuck. If you can take a fairly heavy cut on a piece of 1 1/2" diameter steel and get a good surface finish with no signs of the work 'climbing up' on the lathe tool bit (as would happen if the spindle bearing clearances were too loose), and if you can face off a piece of stock with no center tit, this is a general indication that the spindle bearing clearances are 'in the ball park'.
My own 'druthers would be to check the bearing clearances. Plain bearing headstocks are 'simple but elegant' in their design and there is more to the workings of the plain bearings than one might think. If the lathe headstock uses the 'felt pads' as described in the catalog pages John Oder posted, I'd be concerned as to the condition of the felt. Who knows when, if ever, the felts were checked or replaced ? Worn felt pads or felt pads that are dirty are not going to raise much oil to the spindle journals.
A note about some automotive gear oils: these oils contain additives which may be harmful to 'yellow metals' such as brass or bronze. It is a good idea to avoid using automotive gear lubes on old machinery which may have brass or bronze working parts (such as a worm gear in the apron or bronze bushings in the apron or quick change gearbox).