There are quite a few oddities with oil and boats. you can breath in oil when riding a boat. boats don't float up oil or really interact with oil. The Screen effect when in oil is also removed if you are in a boat.

what buildcraft version? are there any other mods involved?

It looks like boats have a harcoded check for Material.WATER and BlockLiquid (at least in 1.11.2) so we can't do anything yet (at least not until we move to 1.12, and see if forge has methods there)

So does this mean that carrying a boat is a way to save your life if you fall in an oil pit? Interesting...