Why is decarbing in Chrysler and Force engines so critical even while using a good quality TCW-III oil whose main properties among others, were to combat and curtail carbon build up in the combustion chambers?<br /><br />My 1969 Chrysler 20 HP probably through little use from my father had very few carbon build up in the head and piston crowns, even though, in those good old days, straight SAE 30 oil was to be mixed with white marine gasoline (sans detergents) and the carbon build up should have been worse and it is not recommended in the owner's manual as a maintenance proceedure.<br /><br />Please explain, and please indicate me what kind of brand of oil would you guys recommend to use in my vintage and beloved Chrysler 20. I've been using Shell Nautilus, Pennzoil Premium Outboard, Havoline Outboard and lately Johnson Premium XD5 TCW-III?