Few topics in the classic bike world provoke as much debate as choosing the right oil. Not so much for the two-stroke community (who burn their oil anyway) but for four-stroke engines everybody will give you a different opinion, which can be bewildering. It basically comes down to two factors; what viscosity is recommended and whether your engine has a filter element.
Prior to the 1960s, machines typically used a monograde oil, such as SAE 30 or 40. The higher the number the thicker the oil, so the handbooks of older machines often recommended SAE 20 or 30 grade in cold weather and SAE 40 or 50 in the hotter months.
Multigrades became available during the 60s, eliminating the need for different summer and winter grades. An SAE 20W50 multigrade has the viscosity of a 20 grade at low temperature, so flows more easily after a cold start, and the viscosity of a 50 grade at high temperature, so maintaining a thicker oil film.
By the 1970s most machines, particularly the Japanese, were using SAE 10W40 which is still widely available today. This coincided with the sharing of oil between the engine, gearbox and clutch, which is why you should avoid car oils, in case they affect the performance of your clutch plates.
With the right viscosity selected, it’s time to look at the filter arrangement. Until the late 1960s most bikes used rudimentary wire meshes in the crankcase and oil tank to catch large items of debris, coupled with a sludge trap in the crankshaft. This relied on crank rotation to centrifuge any fine debris out of the oil into a machined ‘dead end’ where it would be safely held captive.
The sludge trap relies on two things; periodic cleaning to prevent potentially catastrophic blocking of the crankshaft oilways, and a suitable engine oil. In period, the relatively simple oils available did not hold metallic or carbon deposits in suspension so the sludge trap was able to do its job. Cleaning additional debris from the engine sump plate and flushing the oil tank completed the routine maintenance of the system.
By the late 1960s, paper element filters capable of trapping fine particles, like the filters we have today, were appearing on motorcycles. These were designed to work with new types of oil containing detergent and dispersant additives that dissolved many deposits and kept insoluble particles like soot in suspension, to be carried around and removed by the paper filter. A modern motorcycle oil should be entirely compatible with a system like this.
Products sold as ‘classic’ oils often say ‘low detergent’ on the packaging which means they are formulated specifically to work with the old sludge trap type systems. These are the correct choice for bikes without a paper filter, even the later Triumph twins manufactured into the early 1980s.
Once these two factors have been resolved, everything else is secondary. If you’re racing a British classic you may prefer a sweet-smelling castor oil for its high load capacity; if revving a Japanese classic hard, maybe a synthetic. Apart from that, just remember to change that oil regularly.