If you don't get the fitting and the crush washers aligned perfectly, and tighten the banjo bolt down carefully and to the correct torque, it will seep. The airflow from riding will spread a fine film of oil and that will in turn collect road dust and grime. Been there, done that. There's a tiny recess in the top of the cooler and if I remember correctly, on both sides of the line banjo fitting. The washers must seat perfectly in those recesses to seal completely.
