Within reason, any engine oil will do. It is there so that the air sucked in has to bubble through it and any dust that is present in the air then sticks to the oil. Once in a while it is worth removing the bath off the bottom and cleaning it out in petrol to get rid of the muck that settles to the bottom (you’d be surprised how much it traps), and it is also worthwhile soaking the whole air filter body and flushing it through to remove any dirt build up that is in the mesh.
From time to time the carburettor will flood, particularly when cold starting, and this excess petrol will find it’s way into the oil bath in the air filter. Don’t worry about this it is perfectly normal and doesn’t affect how the engine runs unless the level of the liquid goes above the mark on the oil bath. Just tip a bit out to return the oil to the correct level, a little petrol dilution won’t cause any harm.
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