Quote Originally Posted by Mr. Gen View Post
..silicone oil on synthetic washers, anything else will diesel...
While in theory silcone lubricant is good for synthetic-to-metal contact, in practice silicon oil may well find its way to other parts of the gun, where it can destroy metal-to-metal contact surfaces in short order.
For safety's sake, I would avoid silicon lubricants entirely.

A light smear of molybdenum grease on the sides of the piston seal (not the face), and the spring - as is common practice with modern spring rifles and pistols - will be sufficient.
And if it is kept away from the piston face, dieseling - if any - should be minimal.