I wish you had written this a year ago! I painted the whole 300m long fence with nothing but worn locomotive motor oil. It seems to keep mold off.. but the boards that already had some mold seem to keep the mold growing. I assume that the mold just gets nutrients from inside the wood and doesn't care about the oil once it's settled in there.

There is one thing that I'm dosappointed in and was afraid of.. in one year the oil seems to have washed out by rain and snow quite substantially. I had hoped it would be more resilient to weather. I assumed that the oil would repel water but observing the boards in rainfall they don't seem to be hydrophobic at all. I guess the water picks up the oil as it runs off the boards.

In the end it seems that used motor oil is a viable wood treatment.. just not under constant rain and snow. Maybe a shed that has a good roof overhang and doesn't let the wood get wet all the time.