Jack,
New API SM service requirements limit the use of zincdialkyldithiophosphates (ZDDP.) ZDDP is a highly cost effective anti-wear additive - you can think of it like the "lead" in leaded gasoline. Nothing performs as well, or at as low a treat rate.
The OEMs argue the phosphorus component affects the life of catalytic converters. The 120,000 mile life catalyst requirement could easily be met with a few dollars more of a noble metal, but instead the OEMs have forced the oil marketers to limit phosphorus to 0.08 wt% max. On first glance this may not seem so bad since the need leads to an advance of science. However, the reality is that while new engines are designed around less ZDDP (roller followers, low tension rings, tighter tolerances, smoother parts...) older engines were not and without a rebuild, cannot be completely assured of satisfactory performance. On the issue of backwards compatibilty, the OEM answer was more like "it should be OK."
The proof that there is not quite as much science behind their stance is that the limit is based on a numerical chemical limit rather than a performance specification, as is done with every other requirement in the API tests. There is not a catalyst poisoning test although they have been trying to develop one for 15 years. All they have done is survey what the "old" oils had and reduced the amount from 0.12 wt% in API SG/SH to 0.10% in API SJ/SL and now 0.08% in API SM. Ummm... can you guess what's next?
Flat tappet engine builders have been the first ones to find that newer oils mysteriously have problems during break-in. When I was with Cosworth, the Peterson Publication editors related stories while discussing dyno day engine building contests that they see engines (cam and lifter failures) lost at a far greater rate. From my Texaco days we had a NASCAR break-in oil because the cam lift and closing rates (and the fixed diameter of the lifter) would cause extreme wear during break-in. So I do know that break-in oils will fix the problem. Luckily, once an engine survives break-in, it has always been that it could then use almost any oil.
For now, diesel engine oils are good for break-in because they still have a higher phosphorus amount. But break-in oils for flat tappet race spec series engines will become almost mandatory (if not already) as gasoline engine oils will become unsuitable and in the next API category "upgrade" for diesel oils, they will reduce the phosphorus level there also.
Apologies to all for what will now sound like a commercial, but the fuels and lubes technology company I am part owner of has developed and sampled engine builders with a break-in oil and we have plans to commercialize it. Anyone interested please feel free to email or PM me and I'll be happy to discuss the attributes and/or supply some.