In regards to analysis of the 4 main and 7 main bearings earlier in this thread, it is not a simple comparison. The actual bearing design and the distribution system for the oil would also have a major impact on bearing life. In other words, a poorly designed bearing configuaration and lubrication system on a 7 main engine, may not last as long as a well designed 4 main bearing system.
In general, the 7 main bearing engine, is considered stronger from the added support. Longevity is more a function of how well the engine bearings, and lubrication systems were designed. Mercedes has re-designed the bearings and oil distribution systems on the same series engines from time to time to improve durability. There is a multitued of different styles of crankshafts, bearings and oil pumps for the basic in line 6 cyl. Mercedes engine of the era. We can only assume that this was evolution of improvement in the design.
Major improvements in metal alloys have contributed to extreme longevity of many new modern engines. Silicone enriched aluminum engine blocks, hardened camshaft and crankshaft journals etc.
Does anybody remember the GM Vega? ...............an example of one of the early failures of the aluminum engine block.