My experience in sound change is from using motor oil with different base oil blends in a 650cc Suzuki V-Strom motorcycle, where there is very little in the way that insulates the engine sounds from the rider, unlike a on car or truck.
I did 3 OCis in a row with M1 10W40 4T in the API SM variety, starting at 3900 miles on the odometer. The guys in the motorcycle board speculated it was high PAO + med VISOM + small amts of AN. I thought the valve train sounded tappy with the bike in motion, kind of like a sewing machine, or dare I say it, a minor exhaust leak. I was concerned that maybe the engine needed a valve adjustment, and that the engine/ clutch/ gearbox were hard on oil, because in all 3 OCIs the shifting went downhill after 2k miles.
After I used up that supply, I put in Motul 300V 10W40, supposedly high level of POE & DiE + med PAO + small amts of Gr III. It was a tremendous difference - all I could hear was combustion and exhaust note.
About 2k mi into that first 300V OCI with the maintenance interval was reached for valve check (~15k), and the tech at the dealer told me all 8 valve gaps were right in the middle of the tolerance range, and no adjustment was necessary. He also told some really stupid stuff, like synthetic oil caused sludge and carbon buildup, that it harms engines and makes them wear out faster because it's made out of chemicals and not crude oil, and any veteran mechanic doesn't need to waste time using a torque wrench. And that was the last time I ever went back for any service at that dealer.
While above is my anecdotal experience, the physicist/ engineer mind in me believes that there's more than viscous forces at work between the molecules in the 300V - the ester molecules are polar, have electrostatic attraction forces, and compete for surface adhesion. It seems like causing esters to transmit sound waves would cause them to absorb more of the vibrational energy while positionally displacing/ varying the level of electrostatic bonds to each other, likely converting some percentage of the vibrational motion into minute quantities of heat, thus dampening the transmission of sound waves.
I did 3 OCis in a row with M1 10W40 4T in the API SM variety, starting at 3900 miles on the odometer. The guys in the motorcycle board speculated it was high PAO + med VISOM + small amts of AN. I thought the valve train sounded tappy with the bike in motion, kind of like a sewing machine, or dare I say it, a minor exhaust leak. I was concerned that maybe the engine needed a valve adjustment, and that the engine/ clutch/ gearbox were hard on oil, because in all 3 OCIs the shifting went downhill after 2k miles.
After I used up that supply, I put in Motul 300V 10W40, supposedly high level of POE & DiE + med PAO + small amts of Gr III. It was a tremendous difference - all I could hear was combustion and exhaust note.
About 2k mi into that first 300V OCI with the maintenance interval was reached for valve check (~15k), and the tech at the dealer told me all 8 valve gaps were right in the middle of the tolerance range, and no adjustment was necessary. He also told some really stupid stuff, like synthetic oil caused sludge and carbon buildup, that it harms engines and makes them wear out faster because it's made out of chemicals and not crude oil, and any veteran mechanic doesn't need to waste time using a torque wrench. And that was the last time I ever went back for any service at that dealer.
While above is my anecdotal experience, the physicist/ engineer mind in me believes that there's more than viscous forces at work between the molecules in the 300V - the ester molecules are polar, have electrostatic attraction forces, and compete for surface adhesion. It seems like causing esters to transmit sound waves would cause them to absorb more of the vibrational energy while positionally displacing/ varying the level of electrostatic bonds to each other, likely converting some percentage of the vibrational motion into minute quantities of heat, thus dampening the transmission of sound waves.
