Dive-aholic:
40% is the liability amount manufacturers are willing to give us. Most regs will work fine with 100%. Just make sure when you get them serviced that they are O2 serviced. The main thing is to use cristolube. Some people recommend viton or edpm o rings, but the old rubber ones work as well, even with 100% O2. Yeah, I know, it's not recommended, but I have used them and know of other divers that use rubber o rings with high % O2 without problems. The only regs you have to be concerned about are titanium ones. Again, probably not a problem, but there have been studies done that recommend not using them with any enriched air. They seem to be safe with recreational limits. I wouldn't use them with 100% just because I don't want to be the statistic if there is one.
Actually, this is the first post I have seen in a long time that offers advice I would consider dangerous.
People have done many silly things, and gotten away with it for a while, but to generalize that into advice to others is, in my opinion, a darn fool thing to do.
As has been pointed out by folks who have a lot of experience in dealing with O2 under pressure, if you are dealing with mixes up to and including 40%, all normal scuba equipment will work just fine. That is a safe and conservative statement.
100% oxygen under pressure is an ENTIRELY DIFFERENT ANIMAL.
Pouring 100% O2 into a tank as we do in partial pressure blending requires that the tank, and the valve be cleaned and set up for "Oxygen Service". That means proper cleaning, proper lubes, and proper o-rings.
When you are talking about 100% O2 under pressure coming OUT of a tank, the critical part is now the regulator. It needs to be set up for O2 service because it WILL be exposed to O2 molecules moving fast under pressure. The movement of those molecules can develop enough heat, given the proper fuel, to create ignition. Lest you think that is not possible, you need to know that a well-known cave diver had her hand injured by an O2 fire in a first stage she was working with. Others have been burned and killed by O2 fires.
The second stage is downstream and under much less pressure, so it may not be such a concern.
The up-shot is this. You don't have to be an anal retentive about it, but you need to show some serious respect when dealing with oxygen under pressure. You cannot just blandly say: "Uhh, I know guys who get away with it. You'll be OK!" That is just irresponsible. :no