Retaining wall - preservative paint

Hi
I have an ironbark (hardwood) sleeper wall im building. Ive heard that its wise to paint the backs of the sleepers with some sort of wood
preservative to prolong the life of the wood (I am already aware of sealing the end of the sleeps). I do not want to paint the front as I plan to oil and leave it with a natural wood finish.

Any suggestions as to what products to use? Something fairly cheap would be helpfull as its just going to be covered up with backfill. Sump oil slapped on was one suggestion.


Cheers
Bjorn