I've used it...in the fuel (1 oz. per gallon of fuel), in the crankcase (1.5 oz. per quart of oil), as well as through the vacuum lines....
Can't say I really seen that much "improvement".....but did fix a cold-start "tick" on my 08 Kia Optima when I added it to my crankcase for a 3000 mile OCI with SuperTech oil....
For the fuel, there's better stuff....my 3 favs are:
Marvel Mystery Oil at 2-4 oz. per 10 gallons of fuel
Berryman's B-12 CHEMTOOL at 1 oz. per gallon of fuel
TCW-3 at 1 oz. per 5 gallons of fuel
Due to the pale oil within Seafoam, it does make a good "UCL" - but there are cheaper options available if you are looking for a UCL, particularly TCW-3 (~10$ per gallon for SuperTech) or MMO (2.97 per quart...)