Hi,
There have been few threads on the Forum about fuel and additives etc. and it seems like there is quite a variation as to how individual bikes, even of the same model, respond.
I have a high mileage A65 and my wife has a high mileage A10, both with well over 100,000 miles each travelled since about 1998.
From leaded to LRP to unleaded to E5 and now E10 both bikes have not suffered in any noticeable way. No additives ever used.
No extra valve seat recession, no pinking/knocking and no gummed up carbs so far. Maybe we have been lucky but although lead started to be put in petrol in 1922, octane was still very low and 'pool' petrol would have been the norm for several years in the 1940s and '50s. Even the R3 is not that far removed from the pre WW2 design of our BSAs.
I would suspect that when the M20/21 was designed the fuel available was of poor and variable quality and the bikes would have been set up to take account of this. An M20/21 would probably run on paraffin if necessary.
From what people have reported on the forum and from my own experience, I would say run it on E10 with no additives and see how it goes. Try a tank of super E10 and see if you notice a difference. Your bike may run cleaner because of the additives in the fuel already but it is quite a bit more expensive. There are quite few articles on line and from what I have read, unless racing or you have a highly tuned engine, additives are a waste of money.
So try a few variations and settle on what appears to be best for your bike.
Dave.