Recently I was talking to another MB club member who has also spent his working life as an MB tech. The discussion got around to which are the most durable engines they have produced. He considered the M103 as among the best with only head gasket, valve stem seals (and guides) and timing cover oil leaks being problem areas. He could not recall ever having to rebuild the bottom end of one. Interestingly, he said they made a mess of it when they added the 24 valve head to become an M104. I then suggested that the M102, which was the four from which the M103 was derived, should be equally good. His reply was that they were long lasting assuming that early versions had regular timing chain replacements to prevent their early demise. Like the M103 (and M104), head gaskets were problematic, but not to the extent of the sixes. He did mention some high mileage examples suffering some "blow-by" fuming, suggesting wear in the rings (or bores or pistons) though. This he had not seen in the sixes.
Like most MB engines, maintenance is the key to longevity of the M102. On a 2000km interstate journey, my 190E-2.3 with almost 220,000km showing, used no oil. Prior to me purchasing it earlier this year, it had a top end overhaul for a head gasket failure, which would have at least included valve stem seals, explaining the low oil consumption. This reassures me that it has a lot of life left in it yet.
The bottom line is that the M103 is probably better, but the M102 is still a long lasting engine well matched to the 190E.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.