Consult your regulator manual to be sure. Chances are it will be ok to use for up to 40%.
Regarding fills:
- Partial pressure filling means putting the correct amount of pure O2 into your tank, then topping it off with air. This requires your valve and your cylinder to be O2 clean and compatible. Your LDS can perform this service for $30-40.
- Banked or premixed means the shop orders Nitrox pre-made, consequently your tank/valve does not need to be O2 clean if your mix will be less than 40%
- If your shop uses a Stik or a membrane system, the compressor starts with regular air and slowly raises the percentage of O2 by processing nitrogen out of the air. If you never get mixes above 40%, your tank/valve does not need to be O2 clean.