How old is the bike, and is this the first time you've noticed the problem? How many miles on the bike, and when was the last time the air filter was serviced and cleaned? If the engine has a tendency to have some oil carry over into the air cleaner, it can collect there for quite awhile before it reaches the tipping point and starts to blow out or drip.
While it really shouldn't be that way, Harley's are kind of infamous for oil carryover into the air cleaner, and some are worse than others. The first and easiest change you can make is to run the oil about a pint below full (halfway between FULL and ADD), then see how the carryover is affected. That has cured the problem on many Twin Cams over the years, including mine. If that isn't the problem, then it will take some digging internally. Perhaps the breather valves in the heads are bad, perhaps the drain back holes in the heads are restricted, etc..
Jerry