The evo sportster of that age has the head crankcase breather system
oil mist in the air being forced out through the breather banjo bolts into the air cleaner may condense back into oil when it hits the cold air in the air cleaner
the umbrella valves in the lower rocker cover allow excess pressure from the breather system to vent into the rocker cover but prevent oil from returning back down from the rocker area
however after some time the umbrella valves become perished and require replacing
if your piston rings are worn some cylinder pressure can bypass the piston rings and pressurise the crankcase and forcing any oil in the sump up and out the breathers
I hope this gives you a few potential answers
Brian