Almost a couple years ago, when I asked my dealer to drain the dealership's conventional, *bulk* Valvoline 5w30 from my Camry and replace it with Walmart-bought Pennzoil Platinum 5w30, my engine sounded much different: quieter I thought.
I know this wasn't a placebo effect, because a couple years ago as a complete novice who didn't know squat, I wasn't expecting to hear any difference. I just expected the change to synthetic would bring easier starts in winter. That was my *only* expectation. Nothing else even occurred to me.
All of a sudden while driving with the PP synthetic, I thought to myself, "Hey, this car sounds a whole lot different. What's up?" Musta been the PP synthetic the dealer put in.
I know it wasn't a result of new oil, because I had had one other bulk oil change before, and didn't notice anything at all with the new bulk.
By the way, I do my own oil changes now, as of 2 weeks ago