Change oil. With classics that is fairly short, namely every 5000 km or 3. With motorcycles it was often even every 2500 km.
Nowadays, however, soon 30.000 km or annually. Because a classic “leaks” more along the piston, and in comparison with the current technology has a fairly 'primitive' fuel system, the oil in a classic engine becomes acidic, ages and pollutes much faster than in a modern engine.
For a whole range of reasons, our classics do not like modern, often thinner oil. Just because of the greater tolerances within a classic engine and the too active cleaning properties of modern oil types.
In addition, it is important for older American V8s and air-cooled VW engines that during at least the break-in period after revision be lubricated with a zinc-containing oil. And that is something that not all revision companies know ...
Our advice: give your classic 1x new oil every year. Preferably do that after the last ride of the season.