[Zeke]

Motor oil is a bad idea. Slippery when wet and sticks to your shoes dirtying up your truck carpets or even going into the house after climbing on the trailer.

PT is usually not the best grade so the treatment is offset by the cost of lower grade lumber. I'd go that route as well as it does get beat up.

Let it season a bit and then add a preservative. One that soaks in, that's why waiting a bit.

I like the stake pockets. Makes the trailer useful for many purposes. I'd be on the lookout for some cheap plywood to make sides.