From the Super Mario Wiki
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Oil Slicks are uncommon obstacles in the Mario Kart games. They primarily appear on standard road tracks, such as the Mario Circuit courses from Super Mario Kart, and cause the drivers to spin out when they run over them. As they are rather small and often placed at the edge of the road, avoiding Oil Slicks is not very difficult. Players can also jump (using a Feather) or hop over them. They appear in all but the first Mario Circuit in Super Mario Kart. The Oil Slicks return in Mario Kart Wii (in SNES Mario Circuit 3) and in Mario Kart 7 (in SNES Mario Circuit 2). It doesn't return in the courses appearances in Mario Kart: Super Circuit.
A few Oil Slicks appear on GBA Mario Circuit in Mario Kart 8 and Mario Kart 8 Deluxe. They function the same as in previous games, and are presumably spilled by Mario Motors, which is present outside of the course.