I don't have much on my truck either. I am just going to spray used engine oil under my truck.
My truck does this for me! Except instead of engine oil, it is power steering fluid. Works GREAT!
I fixed the leak 20k ago, and I've pressure washed the thing a couple times, but the oil just won't come off at all. Best rust inhibitor ever!
But seriously, don't intentionally spray oil on your truck. It is bad for the environment (in theory), but mostly, it makes your truck look like a beater and collects dirt.