Oil and a little diesel fuel for trailer deck wood.

[yanmars]

It appears from the comments and experience that the old combos work. I do know if a gunstock gets oil soaked it tends to get the wood soft.

 

[Norm W]

I have a hemlock decks on my trailers. I use oil drained from the two Detroits I have. Some of the oil is around 10 years old. With 7 and 8 gallon sumps I doubt I'll run out of oil soon. The oil is "Black" with soot and the decks look painted. One is about 12 years old and still looks good.