M104 Oil Dipstick O-ring, how important?
 

I noticed mine's kind of worn, am replacing it, but am wondering if this affects fuel/air mix. Is air past the dipstick "unmetered air" for the lamda to compensate out?

The crankcase in the 104 vents into the intake crossover tube, which shouldn't really see a vacuum unless the air filter is packed full of crap. I'd expect to have oil vapors leaking out past a bad oring than to have air leaking in past it. Regardless it's cheap enough to replace it so just do it already:P

Dipstick O-ring has no relationship to fuel/air mixture. It's a good idea to replace it if it's worn/dried out.

I changed mine.

On many recent Bosch fuel injected engines, pulling the dipstick while the car's running will kill the engine.

It's important.

That's why I was wondering, lots of engines will throw a code if the dipstick isn't seated as it introduces unmetered air. No big deal, just wondering if my car was compensating and will need to adapt now that there's a fresh O-ring.

I have never found a day that I regret buying a high-quality O-ring assortment. That once every couple of months that you find a marginal one, pull out the assortment and install a new one, ... worth the savings in time/fuel/mileage alone.