I noticed I last changed my oil using Mobil 1 10-30, not 5-30 as mfr recommended. I live in CA, so don't have subzero issues, but still, is this a cause for concern or not?

Doh !

 

Probably not but if you are worried you could always just drain the oil without replacing the filter and refill with recommended weight oil.

 

You will be just fine. There is not a big enough difference between the two weights to worry about it.

 

Good thing you're in California without the freezing temps. The first number refers to how the oil behaves in cold temps. The last thing you want is thicker oil in cold climates. Also keep in mind that both those weights of mobil 1 don't meet the same specs as the BMW/Mini 5w30. Specifically they don't meet ACEA A3. 0w40 Mobil 1 does however.

 

You're be fine. Just keep it clean.