If your mates big Stu, the fact that he's 17 stone probably has some baring on how quickly the engine revs, and 4psi in the back tyre is equivalent to 2psi for "others"
40 : 1 etc refers to 1 part of oil to 40 parts of petrol.
I don't know the correct ratio for a ty80, but 32 : 1 for bike of this age if its a mineral oil.
If its semi synthetic oil 66 : 1 may be suitable, somebody will be along shortly to give you
a more precise answer.
