Thanks for all the replies. Well, it's been over half a year and now own both the Johnny and Goodfella. We've used cornstarch on both when it's just foreplay but rinse it off prior to internal use with no side effects to either us or the toys (in fact we also own several Fleshlights that we give exactly the same treatment to and that's why we asked in the first place). Oddly enough after researching this, it seems that a lot of websites suggest that you should
use cornstarch on silicone toys since it prolongs their life, prevents the outer layer of the silicone from hardening and absorbs moisture.
And to Mr John - thanks but we're both allergic to the parabens in Maximus. That one lube probably causes more discomfort than we'd have if we left the cornstarch on the dildo! But of course, we DO use a good amount of water based lube (Sliquid Sassy Booty or Probe Thick & Rich usually)
Oh and FieryRed - thanks for the heads up on the medicated powder idea too. Luckily we haven't tried that but we'll still steer clear.