I know tung or Linseed will work as the traditional military oil finish. Every once in a while,rub a few drops in, giving it some love.
Now...TruOil,Watco, etc. To do a complete stock finish job from new,bare wood,they have their own application process. They harden. They work good,I have not finished a wood stock in quite a while.
These finishes ,while they do contain oil, they also contain ,for lack of the most accurate term,varnish of some form. It takes pure oil longer to set up,and it may never set up as hard.
Here is my concern. IF Zastava started with Linseed Oil...No problem! Thats probably what every US Military Rifle that left the Armories used since the Trapdoor through the M-14.. (Or some concoction thats mostly linseed oil)
But think of the wood as a piece of toast. If you smear mayonnaise on it, you can smear more mayonnaise on top. No problem. Oil on oil.
If you start with peanut butter,you can smear peanut butter on top.
If you start with Peanut butter, you can smear mayonnaise over it.
But if you start with mayonnaise, its hard to spread peanut butter over it.
I can't say that you cannot apply TruOil (or Watco,etc) over a stock that was initially finished with just oil,...maybe it works. But I cannot tell you that it WILL work just fine. I'm not sure Linseed oil makes a good "primer" base coat for the finishes that harden. I just don't know enough.
I don't want to steer you in the wrong direction.