Update 2: user covington race reported squealing then opened his impact and greased it, video: https://www.youtube.com/watch?v=Wt9gfgdEPV0
Update 1:The only thing is this bearing grease heats up and will ooze out the front under prolonged impacting....I think any grease you put in will do that because there is no seal on the anvil. You can see an example here https://www.youtube.com/watch?v=grLHK-uB7as The dewalt Moly grease does it less because there is not much inside and it is a thick dry grease. The Impact runs a lot better (In my opinion) after greasing it, it had this strange clack clack sound before that is gone. Obviously the impact performs well with whatever grease they are using.
**DON'T PUT A LOT OF GREASE JUST USE YOUR FINGER**

Before you do this I am not sure if this voids your warranty so take note. I can tell you I ordered this part from dewalt USA and it was at my place in about 2 weeks(which I thought was fast). Maybe you can order it from your local dealer and get it faster but I am not sure. If you want to change the anvil on your Dewalt DCF899 impact wrench to swap to pin detent or hog ring, or just for maintenance or repair it is easy. Just remove 4 screws, remove front nose cone , clean,replace parts,grease and screw back together. Dewalt has easy order online service and diagrams for all their tools you can order screws, motors anything online and Do it yourself(DIY). Notice the grease dewalt is using is i think moly grease and the anvil is totally dry because the grease flew off. Maybe it is not the best grease for this purpose... I don't know too much about grease sorry. Also am I putting too much grease?Not sure about that either..but seeing how much grease flew off on the inside of the cone I thought it was a similar amount I put back. I did use $5 moly bearing grease when putting it back together.Dewalt sells a grease for this purpose called G-2008 Molykote , I was unable to find it anywhere except from them for $65! which was a little too much for my budget. This is the same grease they use in all their drills and impacts.
After watching it seems i'm cranking on the screws real hard but I didn't think so when i'm doing it...so use your best judgement.