I used it for many years with good resuls. My father has been using it for over 35 years now with good results in the oil and gas. He's run full OCI's with it up to the full qt they suggest, as well as 1 pint for the last 500-1000 miles of an OCI, and a lot of other ways. Any time I was around him and an engine he had apart for a gasket replacement, or a modification they were spotless.
We've used A-Rx too with good results, but seem to swing back to MMO. Both work, both are good products. There are many tools that can do the same job, MMO does a fine job with varnish and sludge. Works good in the gas too.
As a side note if dad was running a full qt for the full OCI he generally did it during the NY winter. He feels that is the hardest time of the year for oil, and the engine. The car seems to turn over a lot easier with the MMO.
He freed up a seized mini bike engine for me with it too. I thought the thing would never run again. It took him about a week, but he got it going for me again!
AD