What grade of Oil is best

This may sound like a really stoopid question but bear with me pleeeze.

As I have explained previousley I recently purchased a Discovery V8i with LPG conversion. This is my first Landrover and the first vehicle I have ever owned that needed me to service it (All my other vehicles have been covered by warranties that required dealer servicing).

I have noticed that my V8 is using Oil (I'm sure this is only to be expected from an 11 year old vehicle with 163000 miles on the clock). It doesn't appear to be leaking though as I don't appear to be leaving any oily patches on the ground where it has been stood. Here's the Stooopid Question......... What grade of oil should I use to keep it topped up.

Forgive this noobie if I sound daft but it is a genuine question.

It says in the handbook 10w40, but if its done 163k, i'd use something a bit thicker like 15w50

Many thanks. for both the quick reply and for not treating me ike an idiot. Felt a bit stupid asking the question but having never serviced a car before didn'y know. I went to halfords and saw many different types so walked away and thought I'd better ask the people in the know.

Many thanks again