My oil pressure switch just started leaking after the rally in Vermont last week - ordered a new switch from Bob's BMW here in NH on Saturday, got it in the mail today.
For such a relatively low cost item, you may as well replace the switch (and the copper washer that it should come with) -sometimes these switches slightly seep where it is threaded into the engine case, but when they start actually leaking drops of oil, it often means that there is a crack in the plastic body, and it will only get worse. Plus, you may lose the functionality/usefulness of the switch (telling you when there isn't enough oil circulation). If I were on the road, many miles from home I might try to RTV it up to get by until I bought a new switch, but once I got home (or anywhere I'd plan to stay for a little while), I'd replace the switch.
My switch cost $10.66 at Max's, plus $3.00 postage .