Like Jeepstin12, I've used MMO in my gas and crankcase for DECADES and swear by it. It cleans up and smooths out an engine and unless you have an oil leak, a little does great.

I've used Seafoam the past couple of years when I want to clean the fuel lines and injectors. I've seen no bad side effects from multiple vehicles.

Any time I see it on sale, I buy 4 cans.