NetDoc is right... Silicone oil is bad for plastic, mmmmkay? Stuff like Armor-All provides symptomatic relief for cracking or faded plastics, but it actually just makes the situation worse over time.
My plastic protectors of choice are Vinylex and Aerospace 303. Vinylex is available from auto-part stores, and Aerospace 303 is available from boating stores. Neither contain silicone oil, and neither degrade plastics. I haven't used either on mask seals, but I've used both on car door and convertible top seals with great success. I'd expect they'd work just as well on mask seals.
- Warren