To the OP, be sure to download the RAVE, which is essentially an electronic copy of Land Rover's shop manual. Lots of good, good stuff in there. It's free from a multitude of sites. A good search will bring up a few sources.
It is there that you can read up on Land Rover's oil specs and when and where to put what type of oil in.
Rotella and Delo 400 15w40 "do" make Land Rover's recommendation, but Chazz is right, it depends on climate factors. It's great for the USA's southwest deserts during the warm seasons. I run it when ambients are around 50F to 100F. In the fall I switch to the 5w30 because I"m too cheap to pay more for the synthetic 5w40 and 0w40, which are also good choices.
What you DON'T want to do is just blindly put in oil without reading the spec. When I got my disco I saw a quart of "high-mileage" 0w20 in the back that the previous owner was using to top-off the engine. That viscocity was good for -40 to 0F..... Not too great for Utah summers!
The history of the Rover V8's oil types is quite crazy and a lot of it is based off of internet hype (as Chazz eluded too), there are more important things to worry about than your brand of oil: The cooling system is far more critical, followed by your non-servicable front shaft.
The other issues are hit-and-miss depending on how the vehicle is driven.
I also have to comment on the other poster who said "don't worry about the CDL". He's absolutely correct. If you are doing mild stuff, it's overkill, which is why Land Rover took it out. My target was a mild build that could take on Moab's Golden Spike (obstacle bypasses-allowed), and a CDL would be a better mod than say a roof-rack for that type of build. If you are just camping, you could find better uses for your $$.
Good luck! Discoverys are awesome rigs! Take your time and get a good one!