my take on their viscosity recommendations is that they're looking at operational oil temps. for instance, if they feel that a viscosity of 15 Cst is preferable, they will spec an oil that is roughly 15 Cst at the temps you see. which might be about the case with their 15w50 running 240 degree oil temps, whereas below 225 a 10w40 would be closer.
just a thought.
from studying the oil pressure gauge, i can see how i'd want to run a 40w or 50w oil when pounding on the racetrack in summer (where oil temps can go to 300), but a 20w or 30w if highway cruising in winter (when the oil temps barely hit 200).
-michael