Just to be clear, you should never put 20w50 (nor 15w40) in this type of engine.
But that's what the manual says 15w40.
NEVER EVER trust the owners handbook, it just tells you what at the time were the conventions between rover and oil suppliers.
It says 15w40 because at the time austin rover had agreement with the oil suppliers to stock and advertise 15w40.
Same happened in the late 90ies when they reccomended 10w30 oil, now try to put that in your mini and say goodbye to the 'box!
You want to use something different than 10w40 in your automatic mini, then do it, but be prepared for a gearbox re-build.













