My 2ยข worth....I use the manufacturers recommended (SAE/API/JASO) spec oil. I have never had an engine oil related engine (trans) failure. I have been using Super Tech & Rotella in all of my "conventiional" oil engines, the couple that require full syn, I'll bounce around between brands but always buy the least expensive. I doubt I will ever buy am soil mainly because my Motor Company riding pals swear by it, they have plenty of money to waste too.
FWIW, I bought an a brand new Accord in '93, ran it 250k miles over the next 14 years. I ran the car on walmart super tech oil for the last 200k that I had the car. When I went to perform valve clearance checks at 100k intervals, I found no sludge build up in the valve cover or cam area, it was all clean. In '07 I sold the great running Accord to a neighbor that needed a cheap relaible car, he drove it for another 4 years until the body was completely turned into swiss cheese (NE Ohio weather & salt). The car never "used" oil and never had a problem passing state mandated tailpipe emissions tests.
So, I guess I'm gonna be a hard sell when comes to going for the top shelf stuff.
Oh, getting back to the OP question "Why do we have oil threads?", because opinions are like rear ends, everyone has one and they all stink. I think I just incriminated myself.
