MF, Adding oil to the air filter makes the foam much more efficient - according to the old gregory's manual it improves the foam filters efficiency significantly (they gave percentages but I can't recall or find them now).
Without the oil, they tend to let the dirt a fair way further into the filter and you can see some dirt in the carby throat. That being said, the issue with oiling them with regular oil I have found is that the oil runs down over a few weeks. Then the base of the air filter ends up covered in oil and gets clogged with a thick 'sheet' of dirt right at the base of the air filter - then the engine runs rich and buggers up the bore through running too rich because no one ever checks the filter.
The chain oil is quite tacky - and sticks in the filter once you massage it through. It does still run a tiny bit, but no where near as bad. You can get specific air filter oils like Fab1 by Maxima (which is actually for bikes), but its a pain to clean.
https://www.briggsandstratton.com/n...-in-your-briggs-and-stratton-engine.html
