I can't remember for sure if true oil darkens much, it may a tiny bit. If the wood is really light and you want it darker you will have to stain it first, otherwise just apply the true oil. I've used it a few times and never needed to stain first. Clean up the stock, easy off oven cleaner is a great way to remove old finishes, oil and grease. Spray it on, let it sit maybe 5 minutes then wipe it off, maybe do it twice. Use tap water to rinse it, make sure and remove all the oven cleaner and let it dry out. Sometimes dents in the stock can be lifted by placing a damp cloth over the dent and using a clothes iron to heat the cloth and blow steam into the stock, raising up the wood. Depending on the stock you may want to do some sanding, be careful not to remove very much wood or you can ruin wood to metal fit, also use a sanding block on flat areas to prevent "dishing". Once the stock is all ready for finish just put on some true oil with your hands, rubber gloves are handy. Smear on a thin coat and let dry, I usually put on 4-6 coats. If you get any runs you are using putting it on too thick, but you can remove runs with a little steel wool. I generally rub the stock down with steel wool between coats. If you want a flatter less glossy finish you can put the last coat on with steel wool. i did a 22 that way and was quite happy with the results.
You can go more in depth then I do there is methods for filling pores and whiskering the wood when sanding which probably give nicer finishes yet, but I'm not that knowledgeable yet.
