This would be to Tar Barrel what Kerosene is to Moonshine.
The concept is pretty simple. The pilot injects oil into the engines, creating smoky exhaust. Make the clouds gray instead of black.
Rather than a puff of smoke that does severe damage at a click, this would be more of a leave on and run tool. It would do a moderate amount of engine damage, a light amount of enemy damage, and would lose spots. While on, it would make a smaller stream of clouds instead of one large. This would give the game back the light escape clouds lost with the Tar change.