I have been using ValveMaster lead replacement additive in my 1975 'leaded fuel only' BMW R60/6 for a few months now. I have stuck strictly to the 10cc per litre suggested. The bike does not do a lot of mileage, and recently it's started being difficult to fire up from cold. I checked the nearly new plugs today and found a sort of thick brown shiny varnish coating on them of a type I have not seen before. I replaced the plugs and she fired up first crank. Has anybody else seen this? Is it likely to be the additive? Can any of the Netrider chemists give me clue?