Shell not available - well no surprise as Royal Dutch Shell importing to your country would not make much sense. Your shop is correct, a heavier oil will make a weak pump work harder. What you would want is something 10W40, or 15W40, and rated for use in diesel trucks. Pretty common stuff. The diesel rated oils do a better job of cleaning the engine, in a diesel soot has to be kept in suspension so the big oil filters can trap it. The extra chemicals in the diesel rated oils do very well with these engines.

Testing the pump is a good idea, and if the pump is good, then you would be able to run any Rover advised weight oil, which goes up to 10W60.

 

Thanks a bunch Savanah, I've been working but will try and at least to a oil pressure test.