There is no mystery here. Grasscutter said he/she had done a lot of city driving and idling. Starting in January/cooler temps. The OLM is doing it's job. Change the oil.
So,….is it possible that you didn’t reset the OLM? That would explain you getting down to near 0% from 30% in 3,500 miles. Could it be that simple?
What is this vintage GM V6? You know better than to come in here with that vague description. This is BITOG!