Almost certain to get away with it. I got away with 9 litres in a 60 litre tank. But best to add a lubricity additive, and keep filling up. Never let the level get too low. Some of the petrol could stay there floating on top of the diesel for 4 months before it finally disperses. What you don't want is for the level to get so low that the fuel pick up in the tank sucks up a high concentration of petrol.