There is very important factor of oil stability. Not every 40 grade oil is as good at over 100, the same about 50 or 60. Also tehre is oil wear. Some oils after only just couple hundred stage miles can be 40 or 30 out of starting 50. There are some oils that cope better with being diluted by fuel some don't cope with that it all has to be taken into account in high powered turbo-charged motor. Also the thing that oilman said about oil temperatures. It's not only the matter of oil temperature in the sump measured by your temperature sensors but also about local overheats inside the engine. If oil works in exreme condition oil sqirted into pistons will overheat and loose some of it's properties with each combustion cycle and so on. Quite a complex matter, that's why it's worth it to use top oils, they are quite cheap compared to engine internals