Hi Steve, welcome, great you've got your Land Rover, i use 15w/40 in my TDi and had no problems, Haynes manual states 5w/30 for non turbo, i change the oil & filter regularly around 3k to 4k miles, recommended is 6k but i think it gets really mucky by then, depends how your using it of course mines is not pushed, like me its old and needs regular TLC. I get most of my service stuff and oils from Mike Aitken's, Ladybank as thier close and have good quality stuff, depends where you are of course, get a good quality brand.
Jock