From the Birchwood Casey web site:
Q: How can I increase the shelf life of Tru-Oil?
A: To increase the shelf life of Tru-Oil poke a small hole in the foil instead of pulling it all the way off and store the bottle upside down after opening.
Q: Can Tru-Oil be thinned out?
A: Tru-Oil can be thinned out using mineral spirits. NOTE: Adding mineral spirits will increase drying time.
Guy