I can see little wrong in adding a little two-stroke oil to the petrol of a Scott that is lubricated by either an oil pump or drippers, and think that a 40 or 50 to 1 mix is about right. Perhaps the only thing to be careful of is to not add too much, because it lowers the octane rating of the fuel, which could cause crank-breaking pre-ignition, (“Pinking”,”Knock”,”detonation”), especially in a high compression long-stroke like a Rep or Sprint Special. Some folk do not have a mechanical “ear” for detecting pre-ignition, and modern crash helmets do not help in this regard either. Retard the ignition immediately if you suspect that pre-ignition is occurring. Modern petrol contains all sorts of things like Ethanol and Benzene, that are intended to replace the old Tetraethyl Lead that used to be added to increase the Octane rating in order to reduce the risk of pre-ignition, detonation, ‘knock’, etc.. However not all petrols are the same, and sticking to the top grade ‘Super Unleaded’ stuff may be the safest option.
It is a great pity that the factory at Shipley was not so blinkered against petroil, and for instance, Cylinder-wall oiling was a half-baked, erratic, addition to the faster models, that was easily forgotten by the rider “in the heat of the moment” of a race, just when it was needed the most. Only the 1939 Clubman Special had automatic cylinder-wall oiling, by means of an extra Pilgrim pump. I have long suspected that racing rules specifying “no additives” or “pump petrol only” was another factor in stopping the factory from recommending petroil mix on top of the pump or drippers in normal use, but they DID advise it for initial running-in purposes. Those same rules probably stopped many two-stroke manufacturers from entering the TT in the vintage years !