So, left the car for a few hours. Batt had dipped a bit. Started it up, it did a bit of a weak start (engine kicked over and it started, but not with its normal strength). I let it run a bit, killed the engine.
Measured, went down to 12.0v.
Started again, this time it worked, but was a super sad sounding start. A bit of a delay after turning the key, and the engine sounded like it had just enough juice to get going, but not much more.
So, it is indeed looking like the batt is dying. I just can’t get over it being totally coincidental that it didn’t have any issues, then prob pops up the moment I’m done changing the oil. So weird.
Going to have to head to the dealer and hope it’s covered. Will keep you posted!