The college I attended had a chassis dyno, and I had the same questions, as just to see what the engine was putting out. The car was a 1996 Lincoln Mark VIII LSC. All in one afternoon, we did the dyno testing. To start, it had Valvoline 5W-30 I think...maybe 10W-30. After changing it to Mobil-1 5W-30 (we blew out the crankcase and got an extra .75qt, too), the dyno was showing 4-5 maybe 6 HP, but it didn't show until above the ~4500RPM range. It did seem to make a difference, that was certain, but it could also be something arbitrary, really. Personally, I was excited, and that just gave me another reason to use good Mobil-1!!! The testing was done, so both runs were at same testing conditions, but also coverted to STP.