Castrol 0W30 hard to find? It is available in every AAP at $33+filter for 5qt. Castrol 0W40 has actually more volume sale in Wal Mart than M1. SOPUS doesn’t need SRT 0W40. SOPUS has two 0W40 oils. They could use A3 0W40 oil if goal was just to push M1 out. There is no pushing here. Companies do supplier changes all the time. Daimler is not anymore using M1 but Petronas. BMW used Castrol, then SOPUS and now might go back to Castrol, who knows.
SRT oil might be developed bcs. certain issues/needs around that engine. All of these oils are at cutting edge of technology. There is no way average oil is going to be approved for BMW LL01 considering ridiculous oxidation requirements introduced in 2018. They are cutting edge for given application, engines requiring High-SAPS oils.