How is oil getting on my MAP sensor?

Oil is finding its way on my MAP sensor in my '99 Plymouth Neon and I have no idea how or why. It's causing oil to get on my spark plugs or so my mechanic says. He said that if you don't drive your car on the Interstate from time to time oil will aculate in your cars piping and that's how it got on my MAP sensor. Needless to say my car broke down 3 days ago, got it fixed, now its shaking again.