I finish timber as part of my living and would recommend Liberon finishing oil after first going over the stock firstly with 240 grit, followed by something like 400 grit finishing paper. I did my GRS stock this way using 6 coats of Liberon. It's stood up to lots of wet weather well as well as providing a good durable surface which is a doddle to maintain. Once every 6 months or so, I simply rub another coat of oil on.
I would not recommend linseed as it will not protect long term against moisture and that's not good for stocks. I don't care what GRS or any other manufacturer says, Linseed simply isn't suitable. The factory finish on these stocks I find pretty poor for the cost of them too.