I know someone who put synthetic Mobil 1 in a rebuilt small block Mopar and he ended up with a little less oil pressure and slightly more valvetrain/lifter clatter noise. He tried it as a friction reduction experiment to see if he could get 1-2 more mph out of it at the track to get into the 110 mph club they have for street driven cars. After that one race he drained it and put the standard oil back into it. Not sure it was doing any damage, but it didn't make it any faster.

I had a Ford small block years ago and ran Valvoline synthetic in it. The oil pressure was solid and more than it needed, but the valvetrain was a tad noisy. I attribute some of it to the aluminum valve covers and steel roller tip rockers it had that tend to create a buzzing sound you notice in older cars with minimal firewall insulation.