This is the first time in nearly 20 yrs that I've used a 50wt oil, & I don't remember if my Bug reacted differently when I switched from 2&1/2qts of Castrol 10W-30 to 2qts of Castrol 20W-50 (& 1/2qt of graphite additive). So now in my Saab, I've switched from 4qts of Castrol Syntec 10W-30 to 4qts of Mobil 1 15W-50. My expectations were that I'd lose some engine noise, fuel efficiency & hp. Well, the engine noise is down as expected, but I'm not seeing a change in either fuel efficiency or hp. Fuel use is easy enough to quantify with a pencil & paper, but hp isn't. And this is where my imagination comes into play. I'd expected to see slower acceleration & quicker deceleration due to the thicker oil creating more drag on all the engine parts. Well, acceleration doesn't seem any slower than it already was (3,000 lbs, 14yrs-old, 161k miles, 2.0L 4cyl), & deceleration, if anything, seems to be taking longer than before (sometimes it feels like an auto tranny instead of a 5sp manual!). It's been 11k miles since my last tuneup, & 2k miles since I put in the 15W-50. So again, is it just my 'magination ... runnin' away with me?