I've done a fair bit of investigating this subject by reading the oxyhacker's companion and searching back through about 8 scuba related websites back to the beginning of the sites that are still up and running. What I have found is this. There was a bunch of activity about the practice when sticks first became popular with plenty of worry and gnashing of teeth about pumping elevated levels of O2.
There has been no cases that I've heard discussed where the practice has been believed by the users to have led to premature failure of the compressor. There have been some where the location of the compressor has been led to failures from overheating and some comparisons that show that pumping nitrox slightly raises temps but that has been debated without apparent resolution. Pumping trimix is a different thing altogether and has been shown to significantly raise head temperatures.
There have been many cases where users have reported many years of success with pumping nitrox. The manufacturers have almost all recommended against the practice as would I if I had no control over the percentage of the mix. Many posts from back then and now by Iain have warned against the use of oil lubricated compressors for the practice and I am not cynical enough to claim it is because he works for a company that is famous for oilless compressors but it would be nice if he wasn't the biggest opponent to using oiled machines for pumping breathing air. I, myself opted for the Rix to help to ease my mind. I decided to operate it in my house rather than my shop so the temps could be controlled and the process could be closely monitored. I use an auxillary cooling fan and my head temps at max pressure are 235, 191 and 194 degrees f for the 1,2 and 3 stages.
If this practice were causing a number of premature failures, I believe the years of discussions of the practice would be filled of stories. Instead there is good advice of the ways to minimize the risks and lots of people getting systems up and running. Threads are started and after the first few times running the machine the posters disappear. That's what I did. After mine started reliably producing 32% with no problems for about 1.5 cents per cuft. I just went on to other things and enjoy my bank of EAN32 and the fact that I really never see a reason to settle for diving air again. I promise if a problem develops in this 25 year old compressor, I will let you know.
The fact that a distributor believes that it is a problem and that you must use a specific block for continuous blending or face failure or contamination is not surprising to me in the least. It's not unlike the countless warnings I received from internet posters and service techs about topping off the tank in my Prius. It will lead to blah blah reduced mileage because blah blah. They heard and repeated it so many times it became their truth. OK no problem. I did it on every tank for 245,000 miles and traded it in on my current one that now has 130,000 miles. My wife did the same in hers for 235,000 miles and she has 100,000 miles in her second one. We are still waiting to experience the canister replacements and the dreaded battery replacement and all the other dire warnings we received from the people that were convinced we were idiots. We never had to fix anything.
Now this is way off topic from the original question that was asked but I will still post it because I am that happy about what I have done and experienced with pumping my own gas.
