FWIW, I changed my oil (at my own cost, of course) at 3500 and put in Mobil 1 5w-30. Now, every time I go to the stealer for the scheduled service, I take along 8 quarts of Mobil 1 and respectfully ask them to use it instead of the Castrol dino oil. I've never had a problem with them doing this and I've never had a problem with the oil.
All oils have ratings and Mobil 1 (and I suppose all other syns) qualify to be used in any Land Rover vehicle. I don't understand why your stealership doesn't know this, but I suspect that they are just being stubborn. Is there another stealer in your area? If so, try them out on your next service.