since the very first oil change on my then new 1994 single cam saturn has had nothing but mobil1 synthetic oil in it. i went to get the stuff to do the oil change today and the autoparts store i use had a really good sale on castrol products so i thought i'd give that new synthetic, castrol edge. well i got home and while the motor was still hot, i drained the oil, pulled the filter and put the new oil in. i fired the car up and ran it around the block then checked the oil level again to be sure there was enough in it. i drove up and the wife said that the car sounded so quiet, what else had i dont while doing theoil change, so i started it back up and got out and stood over the engine and listened. it really IS alot quietr and seems to be smoother as well. i dont use any extra additives in addition to the oil, never have. i'll give another report after a couple of weeks to see if the car is still quieter and smoother. i wonder what is different about the castrol edge over the mobil1 or any other synthetic. i used the same viscosity, the same filter i always use as well. i'm going to turn 450,000 miles this tankfull of gas and it still runs and sounds great. i love my car and it seems that it's going to be around for a long long time