Silicone Grease
Silicone grease is commonly used in nerf to lubricate moving parts such as plunger heads, triggers and bearing surfaces. This not only reduces wear but it can also increase power delivery and improve your air seal.
Silicone grease is a better choice than more common mechanical lubricants which are often petroleum based. These will degrade plastic parts and o-rings causing damage.
Each 10g tube will go pretty far as with most lubricants, a little dab'll do ya.
Warning. Not suitable for use on your naughty bits.
Data Sheet
The Material Safety Data Sheet (MSDS) for this product can be accessed here.