I revisited this problem as the damp cloth advice was totally ineffectual on a really dirty keyboard.
It turns out that VM&P naphtha (available from some paint stores or art supply stores) on a soft cloth instantly removed the oily dirt without affecting the numbers and letters printed on the keys and it dried rapidly with little odor. Paint thinner, barbecue lighter fluid or cigarette lighter fluid would work as well, but the former two take longer to evaporate. By all means, try first on a key you rarely use just to be sure it won't damage the printing on your keyboard (mine is a number pad version from late 2009). Since I used a cloth that was only dampened, not soaked, with solvent, nothing got down into the cracks beneath the keys.