I've seen the same on many FE's (and others as well) and concluded that it is blown up into the threads by cylinder pressure. The clearance in the threads may be enough to allow this when the rings etc. are seating or even on decelleration. The fact that you have a good color would indicate that it isn't excessive enough to affect the combustion process. When you drop the throttle the high vacuum is probably pulling some
oil that gets mixed with the gas and is forced into the thread area. If
oil consumption isn't a problem I wouldn't be concerned.