I think Sturmey-Archer recommended a medium weight oil, about 1/4 teaspoon every fortnight, under heavy use. ( I think that is quite a lot. It will run out for a long time if you put in too much.) If the oil is too thick, the pawls will stick. (They will not engage.) Don't ever use 3-in-one oil in Sturmey hubs, because it's part vegetable oil and will become gummy, necessitating a tear down and cleaning. I use lithium grease on the ball bearings and thirty weight in the oil hole ( just a few drops). If it dosn't have an oil hole it is a later version and intended to use grease. The only way to grease it is to open it up. ( see: Sheldon Brown)