A number of high quality turntables rely on sintered bronze bearings, particularly many older turntables such as the Thorens TD124 or TD125.
Sintered bronze bearings are porous and when heated can be made to soak up oil like a sponge. This absorbed oil is then available for lubricating the bearing which gives them a degree of 'self lubricating' ability. However over time this absorbed oil is used up and the bearing dries out so it becomes important to re-oil the bearing periodically.
A badly worn bearing will create much more 'rumble' than a good one and so sonically it pays to keep your bearings in tip top condition.
Recently I've come across people recommending the use of automotive gearbox oil for lubricating the main bearing on turntables. While this may be perfectly O.K. for some turntables it could be potentially damaging for turntable bearings which use sintered bronze bearings.
Older gear oils often contained Sulphur or Phosphorous based extreme pressure (EP) additives which are well known to be corrosive to copper containing 'yellow metals' such as bronze.
Oil testing institutions test how corrosive a particular gear oil is to yellow metals by submerging a cleaned copper strip in the oil at a standardized temperature and for a set time. After the test period, the strip is examined for evidence of corrosion and a classification number from 1-4 is assigned based on a comparison with the ASTM Copper Strip Corrosion Standards. This copper strip test data is available for most commercial gear oils.
Modern gear oils of the type used in manual car transmissions usually contain a much less aggressive form of Sulphur EP additive which is not corrosive to yellow metals at low temperatures, often refereed to as 'buffered' rather than 'active' sulphur. This is why you can realistically get away with using a modern GL4 gear oils in a bronze TT bearing without any noticeable problems in the short term. However that doesn't necessarily mean that they can be considered safe to use.
In a gearbox the EP additives bond to the hard steel gears creating a sacrificial coating which prevents metal to metal wear of the gears. Problems can arise when a hard steel surface in in contact with a soft yellow metal bearing. What happens is that the EP additives form a coating on the yellow metal bearing. As this coating is worn off it takes with it a microscopic layer of bearing and so causes erosion of the bearing surface.
Gear oils with a higher number rating usually contain higher concentrations of EP additives so a GL5 oil has more of these additives than a GL4 oil.
Engine oil would be a safer bet for use in a bronze TT bearing. Car engines have bronze bearing surfaces (e.g. valve guides) so obviously engine oils need to be safe to use with yellow metals. However engine oils also contain other additive packages such as detergents which are at best of no use to a TT bearing and at worst may actually be undesirable. However given that most of us have easy access to engine oil it would be my default recommendation for the average owner.
An even better choice would be a high quality oil of an appropriate viscosity manufactured from a high quality base stock and containing no additives which may be considered harmful to a bronze TT bearing. Good quality air compressor oil would fit the bill perfectly.
If your TT bearing is a little worn it may be beneficial to use a higher viscosity oil than originally specified by the manufacturer. You need to aware though that comparing oil viscosity ratings can be a little confusing though. For example an 75w gear oil is actually about the same viscosity as a 20w engine oil.
Here is a useful comparison chart:
Previously in my own TD125 I used to use a very high quality 20w50 engine oil designed for racing classic minis however more recently I have swapped over to a fully synthetic ISO 46 compressor oil which I believe is pretty much ideal. If you feel a thicker oil may be better then an ISO 68 or even ISO 100 compressor oil may be a good choice.