Castrol ValveMaster Plus - Lead Replacement Plus Octane Booster - 250ml Bottle. If your Norton (or any other classic motorcycle) has not been fitted with new 'unleaded' type valve seats, then using modern unleaded petrol will lead to excessive wear of your valve seats.
Castrol offer Valvemaster Plus to combat this problem - just add a small measure of this liquid to your fuel tank each time you fill up and you will effectively be changing your 'unleaded' specification to 'leaded type - and therefore will protect valve seats.
Note: Castrol make 2 versions of this lead replacement additive, a standard version, and then this version which also has an Octane booster included. I always add this version to my British motorcycles - as I want to provide as high as an octane rating as possible to the high performance engines we are running (SOHC Nortons, sports twins etc), to reduce the possibility of pinking. Therefore I use this in all my classic bikes.
However, should any more encouragement be needed - it is also endorsed by the Federation of British Historic Vehicle Clubs.
This 250ml bottle will treat 250 litres of fuel (1ml to each litre of fuel). The reason I like this Castrol lead replacement offering over other makes is that the bottle includes a clever measuring device at the top - just squeeze the lower part of the container until the correct quantity of the additive fills the top measuring holder, which is graduated with the different quantities of fuel you are filling your tank with - simple!
Always read instructions before use.