Advice vs pros and cons of engine flush

To answer your question, I used to use a product called 5 Minute Motor Flush. I don't know what it was, but it was a clear liquid, and it apparently thinned the oil down. You put the entire quart in and idled the engine for 5 minutes; then drained the oil. It was supposed to clean out all crud, sludge and varnish. This was back in the mid 80s when I was doing a conventional oil change every 3,000 miles. I didn't start using synthetic exclusively until the early 90s. Anyway, at about 100,000 miles my '86 Dodge Caravan needed a head gasket (it was about 1991). After about 30 oil changes (I used the 5 Minute Motor Flush almost every time), the inside of the engine was surgically clean, and the honing marks in the cylinders looked new. Once I started using synthetic oil, I stopped using the 5 Minute Motor Flush, but I think it did its job well.