ive used both motul and elf very recently, and I cant feel any difference, nor see any temp difference...
So what weights were the Royal Purple VS Motul? Which Motul oil was he comparing - 4100, 8100, 300V?
Even things like differences in humidity will see a 3kw difference from one day to the next!
We change the oil so often in these cars that it would never get the chance to go off - except for burning it at track days etc. I dont think the brand of oil makes much difference at all. as long its fully synth you're not going to see a noticable difference if you're comparing oils with the same weight.