It is common for oil and other crud to end up in the MP lines just due to the way they work and where they are connected. A restrictor in the line close to the engine connection is a good idea, both for pulse dampening and for the very issue you mention. Some certified airplanes actually have a push-type drain valve in the cockpit that is manually actuated by the pilot after engine start to clear the line. Many times an inoperative MP gauge can be fixed by disconnecting the line and clearing it out with fluid or compressed air. It sounds like yours has a bunch in it already, so I would consider performing this when you get a chance. Just disconnect it at both the gauge and the engine and spray some brake cleaning fluid through it followed by some compressed air. I do this at every annual on mine. On regularly-flown aircraft, it's not usually a problem. But when they sit for a while the oil and crud can become more solidified and then block the line.
Vic