I ran some Seafoam in the crankcase of a 1996 Saturn I bought. These cars were known for sticking oil rings, causing them to huff blue smoke and use more oil than a bikini-wrestling promoter.
I changed the oil, put in the Seafoam, and drove it normally for 500 miles. After 500 miles, the oil I drained looked like
black paint. I was so impressed that I did it again for another 500 miles. The oil didn't look quite so bad after the second time, but was still way nastier than 500 mile oil should be. The car went from using a quart every 800 miles to using about a quarter quart every 3,000 miles.
That was proof enough for me. Rumor has it the secret ingredient in Seafoam is...
turpentine.