When I ran Auto Rx in the Aerostar about 5 years ago it apparently cleaned the oil pan gasket so well that the gasket slipped out of place, then every time I hit the brakes a pint of oil would spew out the front. $200 for a new gasket installed. I think this was a freak case if it was the ARX or just coincidence.
I am really pleased with the Ranger, 6000 miles oil blot clean as a whistle, just the slightest hint of a ring. The Mountaineer though, big nasty ring after only 2000 miles.