I use it in all of my old garden tractor engines from the 50's, 60's & 70's that were designed to run on leaded gas. Just a cap full when it came in the metal quart cans added to the gas, it lubricates and prevents exhaust valves from sticking. I also add it to the oil for the same reason, and to help keep internals clean.
I first used it back in the 70's in a Ford 1 ton truck w/6 cylinder. I'd asked the machinist who worked the head for me about Stellite valves, and he suggested using MMO. he used it in his racing bike at the strip. He's the one that told me it's great for the older engines designed to run on leaded gas, and it's upper cylinder lubrication properties.
What really made a believer out of me was when a neighbor girl had an older big box store riding mower with a B&S vertical shaft engine would stop running after an hour or so in hot weather but start back up after it cooled down. She called wanting me to come fix it. I just assumed it was vapor locking but when I tried to start it. it turned over very fast like no compression. I pulled the small cover off at the base of the valves and saw the exhaust valve was stuck open. Came back home and got my can of MMO. By the time I got back, the valve had freed itself. Replaced the cover and added a good capful of MMO to the gas and told her to run it and call me if it does it again. She called me 3 hours later pretty happy it never stopped on her the rest of the day. She then got her own can and added to the gas before mowing and never had any problems with it for the next 4-5 years running it. She done flat wore the mower out after that amount of time, so replaced it.
I use it mainly in older single cylinder air cooled engines but will add it to oil in new to me small farm tractors I collect/use to help clean things up and lubricate the upper cylinders. Gotta' believe it helps to free up/clean piston rings too. Some claim using detergent oil in older engines that has only had non-detergent oil ran in them since new will plug up the return ports. I've never personally had that problem.
With newer engines today and oils out there on the market, don't know that I'd use it in them. But it is great for older engines pre-unleaded gas.
