Flint wrote on 12/20/16 at 14:04:19:

C-85 and O-200/O-300 Continentals are prone to having lead deposits build up on exhaust valve stems.  MM helps soften deposits and prevents valves sticking.  I used to use it all the time on my Cessna 150.

There's no documentation that MMO actually does anything in Continental, Lycoming and Franklin engines. It's all "well I've made TBO twice on my engine" type stuff. Well Mr. I made TBO may have run his engine for two hours every day and always operated with the perfect mixture, keeping CHT in the sweet-spot. That's why he made TBO - the MMO was just along for the ride.

I've never used any additive, and in 1,800 hours SMOH have never had a sticky valve issue. Luck? Proper operation? Lycoming? Maybe all three.