Taken from my old BMW.
I changed the oil to run on Mobil 1 5W-30 Turbo Diesel oil for 8000 miles. The engine ran like a dog, oil usage went up and it was very loud, fuel consumption dipped too.
I then changed back to Castrol Edge 5W-30 Fully Syn oil, car idled smooth, used no oil (checked every 1000miles) and fuel consumption improved slightly.
Not sure why it would be a silly question as I do believe there is no such thing, if there was then what is the point of this tech lounge???
Some things experienced are placebo however rough running? oil usage? increased diesel engine clatter.
After a while people get to know how their engine should sound.... I for one do.