The Oil Slick function is something KITT could spray a thick layer of oil out his Oil Jets to cause pursuing cars to slip off the road.
("Knight of the Phoenix")
Oil button from Season 2
KITT'S Oil Jets in action. Put on a little O.J.!
Oil Slick as it was now called in Season 3 and 4
In "Knight of the Phoenix", the Oil Jets were activated by a button on KITT'S arm rest panel. But this was probably done so that actor Barret Oliver (Buddy) could easily reach it. The button was labeled "Oil" in Season 2 and was labeled "Oil Slick" in Season 3 and 4. Oddly enough, Michael only referred to KITT'S Oil Slick as Oil Jets.