ANY quality motor oil sold today, synthetic, hydro cracked or dino have an excellent additive package. The motor oil we have today is miraculously effective as compared to even the very best motor oil of the sixties.
Adding additives beyond what is in the oil is a total waste of money. It REALLY surprises me that your mechanic thinks that Rislone can clean the engine better than Mobil One. Putting in Mobil One is like lowering a miniature cleaning Woman into the crankcase that does nothing but constantly scrub the inside of your engine.
I am curious, did you SEE him pour in this stuff, or did he just tell you that he poured it in. It would be a plausible scam to run by charging someone for this stuff knowing that the Mobil One would clean the engine beyond recognition.
Sorry for the skepticism, but I've seen scams like this run before.
Have a great day,