I have a Savana 3500 5.7 with 165k miles. Having tried quite a few things to solve a lack of power, rich [+15-22 long trim] running and stumble from a stop. I accidentally solved it. My van burns about a quart of oil per 1500 miles and on a whim I added a quart of Marvel Mystery Oil. That day I had a 60 mile round trip and by the time I got home my van ran great. I looked at the fuel trims and they were running between 0.0 and 0.1. It has been a month now and power and fuel mileage are up. No more stumble from a stop. Best $5 I have ever spent for my van.