I guess the bottom line is that for me (and my old BMW) there's no obvious reason not to keep using it. Of course there's also no obvious reason not to keep using the Castrol 0W-40 I bought the last time either.
69GTX: I was in the Wal Mart I walk to and there were 2 jugs of the older M1 0w40 so I got both then submitted the receipt for the rebate. I'll mix with some M1 5w20 EP or.....
This is something I probably wouldn't want to do. The formulations between the 5w-20 M1 and the 0w-40 are quite different. They might as well be different brands. One is very high calcium, no sodium while the other is low calcium, high sodium. You'd end up with a M1 product unlike anything they produce or test.
Based on the latest Castrol 0w-40 VOA, mixing M1 5w-20 and Castrol 0w-40 would result in a more consistent mix (ie both are lower calcium, higher magnesium oils).