The carb on this series has a port that enriches the mixture when the governor pulls the throttle to wide open and that bloww the extra fuel/oil mix back to the intake area. As the carb and throttle shafts wear that extra fuel starts getting back to that intake a little all the time, the motor will run a bit "rich" and you'll get that oil on the filter sometimes. Doesn't seem to hurt anything and there's no way to fix this (that I've found) without replacing that expensive and really hard to find carb. Might be something else going on but that's what I've found on mine.
