I often see vendors selling oil for table tennis racquets.
Why would one want to use oil on their table tennis racquets?
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The oil is used to increase the grip or "tackiness" of the rubber surface of the racquet. You can actually just use ordinary oils (like sunflower oil) but the oils vendors sell are manufactured for the specific purpose of applying to table tennis rubber.
Anyway, you want to use a medium width brush (like something you'd use to paint model airplanes or something like that) to apply a coat of the oil evenly on to the rubber, then let it soak into the rubber (can take anywhere from 20 minutes to a few hours depending on the rubber). Then repeat this process (applying the oil and letting it soak in) 2-3 times.
Advanced players need very tacky rubber to impart the type of spin on the ball that they want as well as to deal with the spins put on the ball by their opponents shots. Really tacky rubber on a racquet should allow you to pick a ball up off the table using just the rubber surface.