To feed lubricant to the required places machine tools often have what look like grease nipples but are, in fact, intended for oil. The writer has lost count of the number of machines he's found with oilsways clogged with congealed grease - so, before going crazy with the grease gun,  check the owner's manual or decide, "Is grease really necessary".

Lubricants for machine tools can be divided into three basic groups: spindles, gears and slideways:
Spindles
Headstock spindles can run in plain bearings - bronze, white metal or even the cast iron of the surroundings - or "anti-friction" bearings such as roller or ball races.
Plain bearings are often lubricated by a thin oil feed through drip-feed oilers or draw up by felt wicks from a reservoir - the latter ensuring that no dirt can get into the bearing of course. For both these application a plain hydraulic oil such as Esso Nuto is often recommended, Myford suggested this for their long-lived Myford ML7 lathe. Another commonly recommended oil for plain bearings is Mobil Velocite No. 10 - while yet another option is actually a motor oil, but a special one. Mobil 1 fully synthetic in a grade 20. The latter is reported as having been used without problems over a 25-year period in a South Bend lathe.
Roller and ball races can be lubricated by either grease or oil. If the former - as used on a typical Boxford lathe - just a plain lithium type will suffice (unless the manual suggest otherwise of course), the grease "melting" as the bearing warms up.

Gears
For headstock spindles that are gear driven (as well as screwcutting or feed gearboxes) from the 1940s onwards makers 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 whilst also being thin enough to be fed through wicks. They are also formulated to resist the corrosion of copper and bronze - a good thing for 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.)

Slideways
In general "slideway oils"  are "sticky" and really intended for vertical surfaces, hence ones that are horizontal can be lubricated with an ordinary oil. Typical slideway oils are Shell Tonna 68 (ISO 68) and Mobil Vactra No. 2; 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 slideway oils too sticky  and prefer a straight ISO 32 - it gives more sensitive feel on delicate jobs.
One interesting oil is Castrol Magna GC 32, a combined bearing and slide-way oil which, according to Castrol is: "….designed for machine tools fitted with combined hydraulic, bearing and slide-way lubrication systems".  However, those so equipped are always large and complex industrial machines.

Oil Classifications
As the classification system for oils changed to "ISO" numbering (International Standards Organisation) - ones listed in an older manual could well be out of date. For charts showing how they compare see: http://www.lathes.co.uk/oil-charts  e.g. Shell Tellus 27 in the old numbering system has been replaced by Shell Tellus 46 (new ISO numbering system) with Shell Tellus 32 (ISO) being is an acceptable replacement. These are  mineral-based, high- performance hydraulic oils, with a Kinematic Viscosity at 40C of 46 cSt or 32 cSt respectively. However, in reality, any quality hydraulic oil that has the correct viscosity should be fine.
An alternative to Tellus 46 is an R&O (rust & oxidation inhibited) hydraulic recirculating oil with an Anti-Wear additive.
e.g.
Mobil DTE-24
Texaco Regal 32
Shell Tellus 32
Gulf Harmony 32 AW
Suitable hydraulic oils such as these are stocked by agricultural suppliers (farm machinery uses such lubricants) and they should have equivalence charts on hand for the various makers such as Esso, Shell, Morris, Mobil, etc.

Some General Points
On older machines gearboxes and other oil-holding elements often leak and, if this is the case, one option is to use a thicker oil intended for the gearboxes of vintage motorcycles. These are blended by Castrol (and possibly Shell) and are designed to be compatible with the bronze bushes found in some older gearboxes - as well as 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 Vintage Motorcycle magazines carry advertisements for such lubricants. Do remember that older machines probably leaked oil when new and, so long as oil is leaking out it means there is some left - it's when the leak dries up that you should worry.
Another solution is to try a thixotropic grease - this is in a gel-like state when at rest but, if agitated in any way, becomes a liquid. It won't necessarily stop a leak - but will almost certainly reduce it.

If an oil reservoir has a sight glass fitted and the level mark has disappeared, or not present, the oil should come half-way up - never any higher. If unsure, check the position of the lowest gear as, with this half submerged, enough oil will be flung around the inside of the box without causing overheating through excessive churning.

One very important consideration is the fitting of clutches inside the headstock. 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.

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 (easily 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 only available in 45 gallon drums. However, as mentioned, it may be that your local agricultural supplier has smaller containers for use on farms or can dispense from large storage containers..