i noticed the piston slap in my 97 5.4 with 231k on it, rite after we got it i switched to valvoline 5w30 synthetic.. now even sub zero temps the noise is nearly non exsistant... as for the oil additives i dont doubt the mileage increase or the temp drop but the additives in products like slick 50, motorkote, and Z maxx greatly increase bearing wear. if you do an oil sample when these additives are used you will see wear metals way above normal. if you do a lawsuit search for these additive companies you will they loose most of them, i think thats why they are so spendy. in my opinion oil companies spend alot of money in R&D if there was a better additive package they would put it in their oil