Not going to quote your post but I'm quite sure you discovered STP works. Every time you used it your engine knocked. If you do it three times it's scientifically valid. But I'm sure the knocking was combustion .. not mechanical .. as FF suggested.
Funny you should mention Castrol 20W50. The guy I bought the little car from was the original owner (his wife) and she/he used Castrol 10W30 for 270K miles. They took it to a garage for all (every 3000) oil changes and the garage put in "Risoline" .. an additive that is essentially SeaFoam. Extremely high detergent. As soon as I switched to 20W50 (w/o the Risoline) my clutch stopped slipping. The rear main seal was probably worn out and oil was leaking onto the clutch. With the 20W50 oil the leaking stopped or slowed enough to allow the clutch to start working again most of the time. Lucky me.
Am I equating the mechanic for my Nissan to Bob Smith? Definitely. Neither engine or engines need or needed the additives and in one case WAY down the road (literally) a problem occurred from using the additive.
But my point is that mechanics are great and we should pay them well for what they do but they are not engineers. Their knowledge is limited and their exposure to science is usually much more limited so it's not uncommon for subjective advice to come from mechanics at times. But I usually listen to them closely but also usually question what they say. Lots of things mechanics said in the past has been thorally debunked now.
North Western Washington State USA