Don't know that there is a recommended thicker oil. If you have the owners manual you can check in the fluid specifications section. I also think it recommends full synthetic oil.

Is it burning it or leaking oil?

 

 

I run all my rivers on 15-40 rotella truck oil always had always will I ran my disco to 190 k and my 2nd disco has 205 k my range rover sport has 160 k none of these cars ever lost or burned any oil and I would change it at 15 thousand miles

 

Two very different motors, 15w40 would likely grenade the 4.4 Jag

 

I run 15/40 in my sport and I have 159 k on it been using it for years it runs cleaner and lasts longer. There is no technical reason why you can't run 15/40

 

Last longer?