Take a good size bottle of bike specific stuff and if it runs out just use whatever is around until you can find a shop that may have something. Anything is better than nothing (usually)

My favorite long lasting chain lubes are Prolink, Finishline Wet, and Dumunde Tech, they all last quite a while and do not get washed off in nasty conditions very easily.

I would bring a spare chain and swap them every 1000-2000 miles depending on conditions and if possible halfway through have a new set of chains and cassette shipped to you along the route. Not sure if that is a possibility or not but you could add a 3rd chain to the rotation if you want to try and stretch it further.