If the finish is still tacky after a month, the first thing to do is strip the finish off to start over, something is really wrong.
The thinner you use shouldn't matter, it should have flashed off in a day or less, wiping varnishes are thin.
The oil could be a problem, but I doubt it, call the artist store and ask them how quickly the oil should dry. I suspect that stuff is pretty expensive, personally, I wouldn't spend the money on it, or the hardware store oil either. The best linseed oil I've used is Tried and True, there are no driers in it and its put through a heating/polymerization process.
I suspect the problem is the varnish. Brush some varnish on some scrap wood and see how long it takes to dry, the label will tell you what to expect, if its taking longer, buy a new can, and check the date.
Tim
