Firstly, Pellgunoil is not silicone. I believe it is a synthetic oil similar to motorcycle fork oil, although there may be some silicone in it.
I know nothing about treadmills other than the clips from 'You've Been Framed'...!
Regarding 'o' rings , it is ok to use silicone on o rings but.....silicone is very bad between moving metal surfaces. So providing the silicone oil does not come into contact with metal to metal you are ok.
Personally, from what you have said I would use a 20w or 30w fork oil in your application, as it is perfectly safe on rubber,synthetic and metal to metal surfaces.
Oh, and a half litre tin of fork oil will fill about 500 pellgun oil tubes.....very cost effective.
Pete.