A lot of old air rifles from a certain period were simply stained and varnished. Sometimes though, this can hide quite good looking wood underneath. Maybe consider what wood you have, Walnut, Beech etc. and do you want to keep the grain showing? Is the colour what you would like or do you want to stain it? I re-finish many air guns and firearms and hardly ever use the same finish twice - I tend to go with what I think will suit the particular gun. At one end of the scale boiled linseed oil/tung oil/danish oil will give a durable finish. At the other, True Oil as mentioned above, can give a glossy finish. Walnut Oil is one of my favourites giving a nice satin finish. What ever you go for, the level of prep on the bare wood will determine how good your final result will be.