They only approve of them to air, but everyone uses them with normal nitrox mixes with no issues, it's a legal CYOA thing since they use a silicone based lubricant in manufacturing. No different than any other regulator, they are just being Swedish.... They breathe very different than normal regulators so keep that in mind, and for sidemount the first stages aren't optimal since they don't have the swivel turret. The sidemount regulator package from Cave Adventurers can't be beat for price/performance and are what I use for my stage bottles *and also use the first stages on backgas with Poseidon second stages*
Edit:
This is in response to your PM which is best left out here for any other comments.
Cave Adventurers - HOG D3 w/ Zenith Sidemount Regulator Package - Marianna, Florida USA - Never Undersold!
Package in question, same basic package as the D1's, just the new design. Slightly different second stage case design, mainly cosmetic, slightly different first stage design, again mostly cosmetic. Slight performance changes in each of these from the D1/Classic, but none that really effect performance that you would notice. These regulators are made in the same factory as those by Dive Rite and Hollis, most of the parts are disturbingly similar, and their reputation is very good in the technical diving world. Many divers, myself included, were plagued with soft parts problems early on, but those have been remedied, and every dive manufacturer has suffered from these types of problems at some point, mainly HP seat quality in the first stage causing premature failure and IP creep. If the extra $100 is worth it to you for the aesthetics, then go with the D3, the main difference is the extended first stage turret which allows the hoses to swivel towards the DIN screw which most diaphragm first stages with turrets do not allow, this was at the request of Edd. For the price they are only beat by the D1/Classics, but the quality is there, combine that with the fact that with any technical diving cert you can take the TDI repair class and become certified to repair your own regulators, it's pretty much a no-brainer to give them a hard look, especially at about half the price of comparable regulator sets.
Beware, that price will not last much longer as the MAP has gone up significantly, that same package is now listing at $979 at Dive Right in Scuba, and that is where Cave Adventurers will be moving as stock runs out.
Comparable packages
M1 Sidemount Regulator Package - Dive Right in Scuba
Atomic M1-$1500
Nomad Sidemount Regs w/SPG - Dive Right in Scuba
Dive Rite XT $1000
Hollis Sidemount Regulator Package | Dive Right In Scuba - Plainfield, IL - Dive Right in Scuba
Hollis- $1200
Sidemount Regulator Kit - Dive Right in Scuba
Apeks- $1550
https://www.divegearexpress.com/sidemount/regs.shtml
Zeagle-$1300
Poseidon Jetstream-$1k *subtract $100 for Cyklon, add $100 for Xstream*
DGX is clearing out some old Dive Rite stock, almost identical to the D1/Classic sets for about $600, discontinued part numbers but the parts kits will remain available for a very long time.
Buying sidemount sets up front will save you quite a bit of money in the long run as the reduced cost on the extra first stage and SPG adds up when it comes time to upgrade. If you are planning on backmount doubles only then the sidemount sets still make sense as you only have to buy the longer HP hose and you can use the extra SPG and one of the short hoses on a stage/deco bottle setup.