Those are hard to get in? Huh?
Peterbilt has a very nice tool that makes it simple. Even without it, its not difficult - you just plug the short end, insert the O-ring sideways, and use the piston bullet tool to work it SLOWLY down until it slides in.
No problem at all.
Getting the old one out doesn't require any great expertise either, just care so you don't damage the groove.
The Mk10 is perfectly happy with a standard 70 duro EPDM O-ring in that location. I own several Mk10s and NONE have ever leaked using that O-ring in that location. The Mk25, despite using the same SIZE O-ring, is NOT happy with it, and will leak like bejeezus at tank pressures over about 3000 psi if you attempt to do that.
SPs "solution" to that is to use a 90 duro O-ring. The solution the problem THAT causes, that of excessive abrasion wear, which is the bug-a-boo of harder durometer ratings, was to go to Polyurethane for it. Since that O-ring is exposed to full tank pressure (its one of the few that is, and the ONLY dynamic O-ring that is), O2-compatability for Nitrox is a real issue.
That's not a solution, its a series of kludges to get around the original problem which they created by putting low-tolerance plastic pieces in there in the first place! And oh, by the way, the "inside" bushing can be inserted reversed (the outer one can't as it has a shoulder on it), and if you make THAT mistake the reg will leak like hades at virtually ANY tank pressure - so much for making it "stupid tech proof."
Oh, by the way, the MK10 is both simpler to rebuild (due to the lack of all that silly garbage in there that you have to keep track of) and breathes darn near as well as the Mk20/25 - one has to wonder if the "enhancements" were more to keep being able to file patents and keep clones away than anything else.