Just because I had a bottle of clean castor oil at hand what I wanted to use before it gets too old. Yes, the model gets very dirty and it was difficult to remove the castor oil from the plane in the cold. Also at winter time it was a bit hard to start the engine for the first flight with this mix but after that it was working reliable. The price of the nitro is so high here that for 5% nitro content the price of the home brew is almost the same as Tornado fuel. In my 4 strokes I have to use 15% nitro (the plane got a bit heavier than I planned) and now it's cheaper to buy fuel than mix it at home.