synthetic is synthetic...and nothing else is. However if you drained your diff and put in some new valvoline you're good...as far as smoother, I've heard the same thing...there are opinions on both sides when it comes to syn vs non-syn...its still an open debate really...and its supposed to be drained every 60k...I'll do it more often b/c it only costs a couple of bucks and takes a couple of minutes!! I really don't think the job of gear lube (royal purple, redline, amsoil) is to "give" you horsepower...don't believe the hype IMO...but as dmeetro has said, changing all of your oil/fluids(diff, engine, tranny) you will get minimal gains......its a question of clean and proper lubrication, not some magic fluid that gives you more horsepower....once again IMO!! In most cases, don't believe what the manufacturer says if seems to good to be true...read what other people have to say!