Beautiful work Alberto. For oil streaks, I use a very fine artist's brush with very little paint on it. Starting at the leak location, I'll whip the brush back lightly in the direction of the airflow. Start light and build up to the desired thickness til you get the desired look. Oil stains are different: I use a thin wash of a black/brown mix and apply it sparingly at the locations where I want the stain to be. I'll let the wash run into the surrounding paint a bit. Need to be carefull with both methods as it's easy to overdo it!