Rover 4 speed and Rover 5 speed (LT77 and R380) gearboxes had integral oil pumps. The 4 speed pumps I suspect were easily damaged by swarf and larger bits breaking from what was essentially an under-specc'd gearbox, The LT77 had I believe an oil pump made from fibre parts that could easily be overwhelmed oil heavier than ATF. The R380 came in around the time of the introduction of the 3.9 Discovery and was built to take the increase in torque for the 3.9.
I'm not familiar with the gearbox fitted to a 2005 Plus 4 - sorry.
