You do know that you will get a different answer from each person who replies right.
So with that said. If the wood is old and dry, I oil as follows, if the wood isn't dry, I varnish.
Inside - Tung oil 50/50 with mineral spirits. I use tung oil as I'm told it doesn't turn dark. Mineral spirits because it doesn't smell like turps and I can apply it inside (with care.)
Outside - Linseed oil 50/50 with mineral spirits.
I add a 1 oz packet of mildewcide in each batch.
And no more then 1 qt oil in the mix, ie, don't put on too much.
Note that it will take some time for this to dry so don't be in a hurry to varnish or canvas.
The statement that linseed oil makes good rot food came from research done by the Forest Products Lab - a place with no axe to grind or product to sell and pretty good credibility. Personally, I don't use it on anything and have no plans to do so. If I wan't a better quality "oil"product That saturates better and dries much faster, I use Deks Olje #1, which is actually a blend of solvent with alkyd and urethane resins. I've never found any oil finishing product that I thought was easier to work with or which held up as well.
No, what ever they sell in the home centers, ie, Menards, Home Depot, Lowes, etc.
They usually have 1 oz packets for addition to paint in the paint dept, usually for a couple $$'s each.
Note, I've also added the mildewcide to the filler.