My old 7.3 was a LOUD engine. You could hear every hunk of metal moving inside. One day I added Prolong oil treatment while it was running and it instantly lowered the noise level. I actually stood back and went "Wow!".

Don't know why, but it did.

 

Sawdust can do that also.
Used car (crooks) used to do just that
to quiet down a noisy gear box.
BTW we are talking the old days.
So much for the history.

Sean
P.S. I am not saying that the stuff is bad. Just an odd OLD point