I have generally used 15w40 Rotella T4 in SBC engines that do not get exposed to starting conditions below freezing. I believe it is a pretty good formulation for flat tappet cam shafts but probably not as critical in newer roller tappet type engines. There are a number of tests conducted by oil formulators required to optain an API License (such as CK-4, or its predicessor CJ-4). For new engine build break-in I typically add an additive to boost the ZDDP as an insurance policy toward reducing flat tappet camshaft break-in concerns.
There is a lot of pressure on the oil indsutry to develop lower viscosity grades (10w30, 5w30, etc.) associated with improvements in engine fuel efficiency with engine manufacturers trying to ensure that those formulations will function in their engines (both current and previous designs).
Straight grades like SAE 30 are getting extremely rare and it is not clear how long they will remain in the market.
