Oil soaked bottom planks

Petroleum oils are generally not good for wood. They don't destroy the lignen, but tends to cause the same thing as water soaked wood, which is it makes it fairly weak and brittle. These types of oils also don't really prevent rot. Just ask any gunsmith that's got some experience. Engine oils also have a lot of things in them, such as; several different metal particulates from bearings, rings and other moving parts, partially spent hydrocarbons and other contaminates from the combustion process, detergents, friction reducers, stabilizers, etc., most of which will not do coatings, nor the wood any good.

All this aside, some oils, typically fruit and nut have been used for generations to help stabilize wood and prevent rot. Most of these types of oils oxidize, so they need to be renewed, unlike the petroleum products, which just seem to soak in continuously, literally coming out the other side of a plank eventually.

I've saved very few well soaked planks, with replacement being the most economical option for most owners. If it's a fairly new oil stain, I'd try to leach out what I could, then stabilize the area with treatment.