Some notes on 420 timing chains:
First and foremost, it is a stretched chain that causes the problems, not a broken guide. The guide breaks when the chain "hops" a tooth on the right head cam gear when it is too long, or it is too loose and "slaps" too much on startup or at idle. The piece of the guide then falls down and gets under the chain and ...... bent valves on the right head.
The cure is to get the stretch checked reguarly -- the "cheap and cheerful" way is to set the cam to TDC and see where the crankshaft pointer is -- more than 8 degrees out means change the chain. Find a mechanic who knows how and check it at least every 30,000 miles or if you are feeling insecure.
The tensioner on all these cars leaks down every time you shut them off -- they do not hold oil pressure. Hence the slap on startup, and the broken guide when the chain is too long. The newer (60x and 20x) engines use a ratcheting tensioner, pushed out by oil pressure but held out by a ratchet, which is fine until you forget to re-set it when putting in a new chain and promptly overtighten the chain and break it......!
Check the stretch on the chain, change the oil frequently, and the 420 will run almost forever -- it is almost exactly the same as the 500/560 other than the single roller chain instead of the double roller chain.
Synthetic oil is only a problem in an engine that already needs an overhaul -- it will leak more, both from the engine and up the cylinder walls past worn rings, and it will clean up all the dirt out of everything, including varnish filling up the clearance on hydraulic lifters, etc. If the engine is in good mechanical shape, wear will be nearly nothing so long as you change it once in a while!
My favorite synthetic oil story is the Ford lubrication engineer who had to use some during a constant run test, found that engine wear stopped while the synthetic was in the engines, so had his new 1965 Lincoln filled with it when the break-in oil was drained. He ran it for 100,000 miles with only topping off after filter changes at 5,000 mile intervals, then had the engine dissasembled and inspected when it didn't break down and still only used about a quart every 2000 miles or so -- for all intents and purposes, NO wear -- lifters looked like new, still had new looking honing marks, no wear ridge, etc, and HE NEVER CHANGED THE OIL! Published in Popular Mechanics or Popular Science in about 1972 when Mobil first started marketing Mobil 1. I don't actually recommend never changing the oil, but synthetic oils really to what they claim.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!