No time to look for links right now, but there was a good discussion of it in the Additives forum. I believe it was on the STP 4 cylinder treatment. IIRC the company reps have said 500cSt for the regular STP and 100 or 200 for the 4 cylinder stuff. There is a VOA on the 4 cylinder stuff at this site and it shows close to 100 cSt IIRC. You probably are better off with Valvoline Synpower Oil Treatment, though twice the cost (also a recent VOA on this site for it).