This is the medium I got:
http://www.ebay.com/itm/8-LBS-Glass-Bead-Medium-Grit-MIL-SPEC-8-70-100-grit-Sand-Blasting-Abrasive-/111672165459?hash=item1a002e0453:g:VocAAOxydgZTKU6c
Bear in mind I only shoot plastic a few times a month, so this is just from a hobby pourer.
I spray my injector's tube after each load with spray PAM, and my O rings are still good.
I do hear the grit as the injector is pushed. It may be scoring the insides of the tube, but I haven't had any problems yet, and I've been using it for two years now.
When I first decided to try it, I contacted the company I bought it from, Bass Tackle, and asked if it would hurt the O rings.
The woman I spoke to said it shouldn't, but she sent me some replacement O rings, just in case I did develop problems.
I've never had to use them.