Question Author
Well, thanks all. I've found the simple solution, courtesy of the bloke who was felling trees on the farm. It is to buy a 100ml bottle of Stihl's 100ml 2 stroke oil. This you mix pro rata with petrol, getting a 5 litre container of petrol and adding one or two of the Stihl bottles accordingly. But, bizzarely, as observed, different engines require different mixtures.There seems no logic to that.