I had an opportunity to put that physics department's advice to work -- I had bought a used 240 and went about doing the usual tune-up stuff ... then I found the sparkplugs wouldn't turn. Concerned about damaging their threads by forcing them (as well as fearing that the previous owner possibly ruined the threads), I started up and ran the engine for a while. Then the plugs came out easily -- a big relief, no thread damage, but just unnecessarily overtightened.

Of course, I always put a little antiseize on their threads, along with other threaded pieces (I even put it under the distributor's rotor so I can lift it out easily -- you wouldn't believe how many folks have reported having trouble with that), but I had to start at square one with this newly acquired car.