I changed my oil a little while ago, draining out Mobil 1 and filling it with Syntec Blend, both in 5w-30. I don't know if it means anything, but my car is quieter now, kinda sounding 'more lubricated'. The all-synthetic Mobil wasn't even that old or dirty, so I don't know why the motor would sound better, especially under acceleration. Hmm...