Automotive waxes are your best bet. Avoid furniture waxes as they tend to have petroleum in them that can soften the playfield paint or cause color changes, also avoid anything that says "cleaner/wax" for the same reason.
I used a pure carnuba paste wax, there are several brands available, any should do fine. In a nutshell, you want some manner of wax without additional chemicals that might cause a reaction with the playfield paint.
D