Shane,
I can't find a 10w-40 on the Mobil website, do you mean 0w-40?
It seems that both the Mobil1 0w-40 and Castrol R 0w-40 are much the same, both are ACEA A3 rated (good as it gets). The Castrol has a TBN of 10 so it should be good for extended mileage. The Mobil didn't have a TBN in the specs on the website, I'll send em an email.
The Mobil runs a bit thicker at high temps, but there is not much difference. Sulphated ash for Mobil was 1.2, not shown on the Castrol website.
I am buggered if I know how anybody can compare oils, when each company publishes different criteria, and chooses to leave out others.
cheers