Tuesday I was supposed to fly to Sioux Falls to pick up mother (patient) and daughter (support) pair and deliver them to Rochester, but it didn't quite turn out as planned. The original schedule had them arriving at KFSD around 5:30 PM and I got there around 4 PM to have a minor avionics issue resolved (the shop that did my recent upgrade is there). That part of the plan went OK except that I got a call before I left work for the airport from one of the other pilots (this was a 4 leg mission) saying they were a couple hours behind schedule from the get go. That was no big deal as I knew the FBO would lend me a car to go out for dinner while I waited for the third leg pilot to arrive. After eating I returned to the FBO and started reading a book. A little before 8 I got a call from that pilot who told me he had experienced a mechanical problem (oil on the windscreen and smoke in the cockpit) and had made a precautionary landing at a small airport with absoulutely no services out in the middle of South Dakota. He was hoping I would be willing to meet him there and finish the mission from that point. I took on some extra fuel since the 2.5 hrs fuel I had left didn't seem sufficient for the hour flight out plus the 2 hr leg from there to RST and there wasn't any fuel available where they landed. I ended up taking the two pax plus the pilot to RST and then delivered the pilot to his wife who picked him up from my home airport (he lives on the south side of the MPLS area). I got a message from him today indicating that the problem turned out to be a crack in one cylinder that was dumping oil on the exhaust so it seems like the precautionary landing was a wise choice.