When I was introduced to seafoam, I was hooked. Used it as a short engine flush (prior to service) on a neglected pre-owned loan/lease buyback....after about 400 miles, the oil came out extremely dark. So, I changed the oil, and let the car sit overnight (let it, and the new oil cool off), the next morning, I added the recommended dose of 1.5 oz. per quart of oil, at the time, my Subaru took 4.5 quarts in the crankcase, so I measured out about 6-7 oz of seafoam (like half the can), and put that in the crankcase, drove the car as normal, primarily city driving though (no high load, heavy acceleration at the on-ramps......) for a good 3k, and had the oil changed again, that change, it didn't look nearly as dark. It was still dark, but more of a golden/brown rather than a thick "black" to it.....
Didn't feel a third dose was necessary after the 2nd oil change, but I've heard of people doing it that way. Seafoam is safe in the crankcase but only for 2-3k miles MAX. The main benefit I think is the alcohol that loosens up the gunk, gets it suspended in the oil, where it then can pass through the oil filter.
My latest gems though, I've used MMO, with similar success stories...generally, 1 pint in the crankcase when I change the oil, every time, for anything less than 5 quarts. If the car has a 5 qt capacity, swap one qt of MMO for that one quart of oil (so you only add 4 qts of motor oil, 1 qt of MMO).