The Oil Slick is a pickup that, when used, drops a puddle of oil behind the car. Anyone that drives through the puddle will lose all traction on whatever wheels pass through it for a short time. After 30 seconds, it disappears from the track.
Oh, that's just wrong...
The best place to use an oil slick is in anywhere confined or narrow, preferably with a turn up ahead; racers will be forced to drive through the slick and won't be able to turn.
Additionally, dropping an oil slick anywhere where there is little traction normally, such as the freezers in Supermarket 2, is just plain mean.