I switched to shell rotella synthetic a few weeks back and i changed my oilly air filter... there is now no oil in my air filter...

the engine vent system on top of the valve cover pipes oil vapors over to the air filter housing for cooling and so it will turn back into liquid oil... older cars produce more oil vapors because of blow by the rings... and the system becomes over whelmed with liquid oil and you get oil in your air filter...

synthetic oil does not vaporize like the regular stuff... so my air filter is clean..