I've used Castrol 10W-40 a 2002 Discovery, but noticed a definite "bog down" or "drag" effect as opposed to the 10W-30. It's to be expected.
I did notice, however, that it was not as bad when running Amsoil's 5W-40 European Formula. It was a bit quieter and smoother than both of those oils. Felt like it protected much better than the 30 wt as well. Try it out.
Stay away from high-wearing Mobil oils... For the price, you can get a better oil. Check out some UOA around here to see what I mean.