It is easy to experiment, just dribble an oz. on the grass and keep looking occasionally to see what happens. I think FAI fuel will ruin the grass too, as will gasoline. We do pit stops on pavement and grass, and cool the motor by squirting fuel over the cylinder to cool it off, and fill the tank fast, and it squirts all over, making a mess on the ground and an oil spot on the pavement. It does not ruin the pavement, but is ugly. If you fill your plane carefully, and don't idle needlessly and adjust the needle or break the motor in on the grass for instance, then it will not be an issue. That video is a four stroke with the exhaust way up in the air. I would doubt there would be a problem there. Nice video too. Smooth flying plane.