Google is one's friend, especially wrt this MT80 conundrum.
"RESTRICTED
I. – REASONS FOR INTRODUCTION OF M.T.80 PETROL
1. Certain American armoured fighting vehicles, in use both by British and American armies, were fitted with de-rated aircraft engines which required a fuel with a minimum octane rating of 80.
2. After due consideration, it was decided that it was impracticable to stock two different octane rating fuels within the army, and it was, therefore, decided that the basic army fuel would be of 80 octane rating and that this fuel would be the standard supply for all types of vehicles within the British and American armies."
Good discussion on it at this link:
http://ww2talk.com/index.php?threads...of-mt80.58510/