Mario passing by a nearby Oil Slick. See that black puddle by Baby Luigi? That's an Oil Slick.
Oil Slicks (also called Oil Puddles or Oil Spills or simply Oil) are a hazard found in various courses in Super Mario Kart. They spin out any racers unfortunate enough to drive into one. Oil Slicks are commonly found in Mario Circuit 2, Mario Circuit 3, and Mario Circuit 4. They are scattered around various points of each track. In Mario Kart: Super Circuit's remakes of the courses, all the Oil Slicks have been removed.
Oil Slicks also return in Mario Kart Wii and Mario Kart 7 in the games' remakes of Mario Circuit 2 and Mario Circuit 3 respectively, in these games, the spin-out impact from the Oil Slicks has been drastically reduced and it is far easier to recover from a spin out.
Oil Slicks are also seen in Mario Kart 8/Mario Kart 8 Deluxe in the games' remake of GBA Mario Circuit. They've been added as an additional hazard to the course despite not being in the original.
Oil Slicks are also seen in various courses of Mario Kart Tour.
Neo Bowser City features a similar hazard to Oil Slicks called Puddles. Despite the different type of liquid, they work in the same way as an Oil Slick.