Regarding BSA Airsporter S & Mercury S models, I agree, nice walnut is by far the exception rather than the rule. I have seen a few really nice examples, but the majority are nothing special.
As to the pre-war BSA rifles, overall wood quality seems higher and I think many of those already had an oiled finish so tend to look better than the varnished equivalents.
In terms of refinishing, if a rifle is in good honest original aged condition I probably would not do any significant refinishing, but, if it something a bit more beaten up but still in good working order then I would consider restoring it.
If it is an ultra rare model and in original condition of course it should not be refinished, but, there are also seriously abused and/or neglected examples of even the rare models where there is nothing to lose by restoration.
I am not a full on collector, I like shooting the airguns I have.
In regard of the Airsporter S, there were already scrapes in the factory finish, some of i was starting to peel and it was looking scruffy compared to the practically unblemished metalwork, so I refinished it and doing so really brought out the grain of the wood.
As to the rear sling swivel stud, I bought the rifle second hand from its original owners, the swivel stud was already fitted.
It isn't where I would choose to put it if I were fitting it myself, but I am not going to change it as whilst I am passable at oiling stocks, I am no woodworking guru and am not confident of doing a quality repair on the hole the swivel fits into.
In terms of oils, there is quite a selection in the UK market, both dedicated gunstock finishes and general wood finishing oils.
My own personal choice for finishing is to start with an Alkanet oil blend to give colour if required, then just good old fashioned boiled linseed oil applied with as much patience as I can muster.
The right hand side of the Airsporter S butt, the lighting is a bit off, but it shows the figuring:
