Hi Wilf,
My Tremec T56 6 speed has a label on it specifying Dexron 3 ATF. I don't think ATF is going to hurt the life of the trans. The conditions in an Auto trans are a lot hotter and unfrendlier than a manual box.
Transmission design and materials have come a long way since the Rover box was built. Also the synchro rings on boxes like the Tremec are mostly made from organic materials like Kevlar rather than brass. So perhaps they don't need the heavier
oil? (I don't miss the smell of that EP80/90 gear
oil either, yuck!).
ZZZZZZ
A couple of banana peels in that gearbox of yours should fix all the nasty noises (an old used car dealers trick).
Cheers