Peter. . .
Thanks a million for your info on timing chain/guide problems and on the use of synthetic oil. My chain was stretched 10 degrees when I had it replaced, so I guess I was lucky to catch it in time. I've been of the understanding that rail breakage was the primary concern but now I understand from your perspective that it is chain "slap" that results in rail breakage which in turn breaks or jams the chain causing valve damage. So, I guess I should feel confident that the rails in my car are in good shape as the mechanic reported when he replace the chain and tensioner.
Also, I'm going to switch to Mobil 1, at my next oil change. I'm hoping and planning on having my '88, 420 SEL for a good many trouble-free years and a couple more hunder miles. It's a great car.