Does the 1st gen LaCrosse store oil change reset mileages in the ECM/ECU? Like if the owner doesn't have oil change records, is there a way to reconstruct this data from data in the ECM?

 

I don't believe it stores that info.

 

Probably not.

 

Just like a doctor does, he keeps a file of everything you get done. And, the same can apply to your vehicle. Just take a sheet of loose leaf paper and note down ,
Date
Mileage
What was done
By whom and cost if you choose.
This info can be a great help if past work is needed to be known,
AND is a good selling feature in the event you decide to sell or trade the vehicle.
Never any question of what or when, can be kept in the vehicle , if needed when away from home if any repairs are required.