i dump a bottle in the gas tanks of each of my cars every year. i dump it in when the tank is at about 1/4 tank, then drive for a while and top it off like normal. i gained 2mpg from one tank to the next on my 200,000 mile 94 caprice doing that.
never dumped it in the crankcase, tho... maybe i should try it on my clattering 97 Cavalier..