Buy a 100% carnauba wax. It should be hard and softens when rubbed, then sets hard again giving the best protection to your PFs.
Some of the other additives are bad and are for removing impurities from car finishes.
I use Mothers Carnauba Wax its great stuff but very expensive, but lasts a long time
+1 with Nick and Matt. I think, that Mothers sell both a tin and bottle (liquid) form. Go the tin, as it contains no additives etc. Very very good protection for your playfield and safe on clearcoated/restored fields etc.
http://www.mothers.com/02_products/05550-05750.html
