I'm running my French spec '85 Diesel Land Rover 2.5NA (~5K miles on rebuilt engine) on M1 5w-50, occasionally 15w-50. LR spec'd heavier oils in the day. I wanted synthetic and the vehicle sees some cold weather. M1 5w-50 carries diesel specs, has higher than current average Zn and P levels, and I run it in other cars so I always have a lot around.
Nice truck BTW.