I've been experiencing low oil pressure and put in a new sending unit. Now the press is even lower - low enough to turn on the guage light. FasFiero told me that the tube may be coked (over 200K miles!), which could cause a low press condition at the sender. Is it a difficult job to pull the tube and clean it out?

Before you pull the tube, consider that some of thr replacement oil-pressure senders are pretty crappy. If you hook up a mechanical gauge where the sender is, you might find that you have adequate oil pressure. If, in fact, the pressure is low, then pulling the tube could be your next step.