I used lacquer thinner on two performance cars to clean the cats prior to NY state inspection. Worked like a charm since the cats did not give a check engine light, but were taking a long time (pending codes- ot ready). I only used a quart. How often can I do this? Once a year? Catclean days 4x year for their expensive product. As you know a car like a GT500 or a GTR has soot build up after some performance driving.