This is the best oil for diana 350 Magnum?
Best oil for what part of the gun? What are you trying to lubricate?
can this be applied to a pcp fill probe so it goes inside the cylinder when i pump it to prevent corrosion without the possibility of oil combustion causing an air reservoir explosion?
Yes, but the Hill grease will be better and last longer: Hill Hand Pump NLGI 2...
is this oil safe to use on N/P break Barrell rifles like the Benjamin trail N/P rifle
this ok for pcp air rifles if not what do you recommend?
If you are looking to lubricate o-rings I would recommend using some silicone grease. This would not be my first choice. Check out the Hill Pump Silicone grease.