i think most oil additive works with the principle of reducing friction in the engine, unsurprisingly adding these additives do help in increasing the power of the engine while reducing excess wear at the same time. some work also work by sustaining the life of the oil by slowing the deterioration of the oil
i got mine at ENEOS for RM45, that's the retail. I'm running on 198k mileage mivec 1.6 and it does seem to help though i also use petronas semi synthetic oil...idling is smoother and the engine feels lighter, after about 3000km the engine still feels light though the idling has roughen a tad bit...good stuff, considering you only put in 30ml into the engine...
next up is Bardahl engine treatment, i've heard it is their forte of the brand...
forgot to mention: i think these friction reducers really do help with old engines but not so much for the new ones (new technology bla bla bla, new manufacturing process and tolerance) anyway just look at the service intervals of new cars nowadays 5k to 10k...rarely hear the 3k now...much of it is due to the oil being used, but it also reflects on the new technology in the engine itself :)