I love the Mill wax, I use it on ALL of my em's . I would not say it has ever done any damage to my playfields or plastics.
I agree that a nice pure carnauba does a nice job as well I wholeheartedly stand by and would recommend Mill wax to other pinheads. I can only speak for my use of this product on my EM's and early SS pin.
Mill wax has always worked well for me but as always to each their own,use what make you happy.
Brian
Madison hts MI
586-344-0721