You can also thin it with denatured alcohol. 1:4 parts alcohol to Tru-oil. Any more alcohol than that and it gets too thin, and takes forever to get a finish on the wood.
The main thing to remember when putting on oil is to STRETCH the finish as far as you can with your fingers. The stuff will come off your hands with alcohol or any modern gun solvent, or paint cleaner. So, use your fingers to spread the oil on. 10 thin coats works much better than one thick coat. On a half stock, I can usually cover the entire stock with less than 10 drops of Tru-oil. A full stock may take 3-5 more drops of oil per coat, depending on the length of the stock.