Back in the day when the Rover V8 was a new engine in UK , it was fed on a diet of 20/50, and one of the UK's best known engine builders of this unit still recommends it today.
However you are probably aware that this is a "flat tappet" engine , and needs quite high loadings of ZDDP. It is probably more important that you choice of oil contains high levels of this additive than which grade of oil you choose. I have used a Millers Classic 20w/50, but the dealers tend to use a 10w40 semi-synthetic.
The gearbox oil is another area of debate. This box is incredibly strong and it is probably more important that there is sufficient oil in it , rather than worry too much about the grade. Originally we all used a 80 or 90 weight gear oil, but if the box is baulking a lot some users went for a straight 30 engine oil to decrease the fluid friction, I believe that some modern ATF fluids have also been used with success, but I have not tried them.
