I would suggest running the Pennzoil Platinum oil product for your application. It is a highly refined mineral oil based product that helps Auto-Rx clean faster than a full synthetic. About overfilling, 2 bottles of Auto-Rx constitutes roughly 2/3rd of a quart of volume. That would normally not constitute overfilling. Or you could just leave the oil slightly lower than the full line before adding the Auto-Rx.
You will be only running the two bottles of Auto-Rx for 2500 to 3000 miles. A good quality Group III oil like Pennzoil Platinum would work fine.
If you run the Auto-Rx with Mobil 1, then just up the cleaning mileage to 3500 to 4000 m iles, as it cleans a bit slower in group IV oils, such as Mobil 1.
