Oil blow out the breather

This is a common problem on all sportsters, they have an oil blow by that dumps it into the air filter. I have found that if I run about a half quart low the problem goes away.

Too much oil; bad umbrella valves in rocker covers;

Failed umbrella valves are common on bikes over 10-15,000kms. Solution is fit an inline breather valve system, to cure the problem without a tear down. Overfull oil tank is number two, as stated above. Modified ignition and or altered cams can also contribute.
Increasing crankcase blow-by vapour volume if it's a high mileage engine, is very likely. Try Lucas stabiliser if so. You also can check case air pressure for this. A failing oil pump leading to wet-sumping is also likely on a high-mileage engine. Good luck.

If you are fairly handy with spanners you could try fitting a kawasaki engine and frame. Much better!

Excessive crankcase pressure is always the cause of oil blowing out of the engine breather. It can be caused by, combustion gases bypassing the rings(ring, cylinder, or piston wear etc.). Excessive oil in the oil tank. Sticking oil check ball(wet sumping). Wrong type or weight oil.

An overfull oil tank will do it. Even if not overfull just about all harley's will blow oil out the air filter. Unless there is a bunch of oil coming out, it's not a problem

What I've found is that the air filter eventually gets saturated and then starts to leak through. The level in the oil tank doesn't seem to matter. If you change the air filter the problem goes away, until the new one gets saturated and then it starts all over again. My plan is to get a K&N air filter, which you can clean yourself and save having to buy a new air filter every 4,000 miles or so.