Sounds like might have been an improper balance of seal swell. FWIW, M1 0W-40 stopped the valve cover leak on my M5 too.
A lack of failure really isn't a preponderance of proof though. I used a variety of different gear oils in our Expedition diff, towed heavy with it while not being kind to it, never had a failure. I don't consider that a testament to the quality of those gear oils
I know the load of 10,000HP is nuts, but it's also only for 3-4 seconds. I'd think that again, the 24hr cars might be a better benchmark, due to the extended use, but OTR truck diffs are another thing to consider, given the load they are required to deal with, for long periods of time. That's why my current stock of gear oil is Delvac 1 75W-90, which was used at the heavy truck dealer my buddy worked at.