My F56 has maintenance to 100k miles, and I have in-between interval oil changes done.

Since I started a new job, I took it today to a mechanic close to work that has a very good reputation and specializes in European cars.

I explained to the person at the counter that I didn't want the counter reset - only the oil changed.

Well, the mechanic reset the counter.

How is this going to affect me when I bring it in for its dealer oil change in 4k miles? Am I going to have to pay for it? Or will they know? Am I able to undo what was done by the mechanic?

 

Show receipt for oil change to service department when you go for next oil change. Let them look back on the service history to see when the last time they changed the oil and they can do the math if needed.

 

It shouldn't change the internal counter.
The one the dealer resets during the service.