Some Notes on Machine-tool Oils
If the lever mark has disappeared, or not present, the level should, almost certainly, be half-way up any sight glass. If unsure, use enough oil to half-submerge the lowest gear in the housing - this will give enough "oil fling" to lubricate everything without overheating through excessive churning.
For use in many types of machine tool - including the geared headstocks and screwcutting gearboxes of lathes and milling machines - manufacturers from the 1940s onwards often recommended an "anti-wear" Hydraulic oil such as Shell Tellus 27- although some did specify a plain mineral oil ("Vitrea" in various grades, without any additives). These lubricants can deliver their specification performance while also being thin enough to be fed through wicks - a common fitting on many older machine tools, and used to draw up oil but leaving dirt behind. They are also formulated to resist the corrosion of copper and bronze - a good thing for the bronze bearings commonly found in headstocks, aprons and screwcutting gearboxes - and not something an "ordinary" oil will have, and certainly not a motor oil (as a tip, modern motor oils are no good in older-type machine tools - they are designed for high shear rates, high-pressure pump feed and the sort of thin films that hold bearing surfaces apart at high r.p.m.) Car engine oils also contain detergents and dispersants intended to that keep particles of dirt, metal and combustion by-products in suspension. This is ideal if a filter is provided to trap the nasty bits but not so good otherwise. Oils intended for machine-tool use allow the particles to sink to the bottom of a sump and (hopefully) stay there. The writer has dismantled more than one geared headstock to find an almost solid layer of encrusted dirt, metal shaving and "bronze dust" in the base.
Avoid the hydraulic oils intended for automatic transmission systems (ATC). These have been known to cause trouble in some machine-tool applications.
Cross-reference for Makes and Grades
According to a cross-reference sheet, Shell Tellus 27 (old numbering system) has been replaced by Shell Tellus 37 (new ISO numbering system) with Shell Tellus 32 (ISO) being an acceptable replacement - these being mineral-based, high- performance hydraulic oils. However, in reality, any quality hydraulic oil that has the correct viscosity should be fine - a popular choice being Tellus 46, an R&O (rust & oxidation inhibited) hydraulic "recirculating" oil with an Anti-Wear additives.
If your machine-tool gearbox box is leaking with poor or non-existing sealing arrangements, another option might be a light grade of motorcycle engine or gearbox oil. These are made by Castrol (and possibly Shell) and designed to be compatible with the bronze bushes found in some older gearboxes as well as various oil-immersed clutches. They do look to be good, safe alternative, especially since they are sold in 1L and 4L packs. If details are hard to find, most magazines about vintage motorcycle carry advertisements for such lubricants. However, as one wag put it, the time to worry is when the leak stops - at that point you've run out of oil.
One very important consideration is where the headstock has a multi-plate, oil-immersed spindle clutch. These are usually by Matrix, Ortlinhause or Hurth and must in no circumstances be exposed to ordinary motor oils. Only a hydraulic oil must be used.
A dedicated slideway oil is Shell Tonna 68 (ISO 68). However, on small lathes, below 5-inch centre height, it may be found that a slideway oil with a ISO 68 viscosity is too heavy and the machine will function perfectly well with a ISO 32 grade. Personally, I find the slideway oils too sticky and prefer a straight ISO 32 - it gives a more sensitive feel on delicate jobs.
Plain Bearing Headstocks
Plain bearings often have wicks to deliver a supply of oil upwards from a reservoir inside the casting. If you replace the felt it needs to be the correct grade - or at least a dense type - for it has to transmit the oil by capillary action and regulate the flow while also acting to filter out any dirt. Industrial-grade felts in various sections and thicknesses are still available from engineering supply companies and it pays to look out the correct type, not a household grade.
When dismantling wick systems be aware that there is often a spring pushing the felt upwards - a careful examination of the headstock exterior might reveal a small hole into which a piece of wire can be inserted, as a temporary measure, to hold the spring down while the mechanism is reassembled.
One problem with machine-tool lubricants is that the major players - Castrol, BP, Shell, Esso, Gulf, etc., keep changing the names and specifications of their oils. Their "Technical Industrial Departments" (sometimes found on the Internet) are always willing to make a suggestion if you can quote something about the operating circumstances: the H.P. transmitted, the type of gears and, if possible, what the original oil was. Armed with this information you can then contact the sales department, who will inform you (as you've discovered) that the oil is available only in 45 gallon drums. However, it may be that your local agricultural supplier has smaller containers for use on farms.
Unless specifically recommended by the maker for a particular part avoid using grease on any part of the machine - it will attract chippings and turnings like a magnet and do little good. What appear to be a grease nipple on a machine tool is invariable intended for oil - the exception being headstock spindles running in ball or roller bearings where, generally, grease is used.
For a discussion about which oil to use see: http://www.practicalmachinist.com/vb/greatest-hits-and-links/lathe-milling-machine-lubricants-110671/