I hooked it into a crude oil tank and it still complains that there is no fuel. I hooked your gas-fired boiler to crude oil and it works just fine, so what's the difference with this furnace?

You're probably hooking up the crude oil to the ingredient input instead of the fuel input. Unfortunately, the game engine does not generate an "in arrow" at the fluid fuel input. The input and output arrows are only generated for the ingredient input/outputs. To hook up fuel, run your fluid fuel line up to either side that does not have an arrow. The engine doesn't mark those. Nothing I can do about it. If you accidentally loaded oil into the ingredient chamber, expect the furnace to output some solid fuel once you power it.

What can I produce with this furnace besides regular smelting recipes?

The fluid ingredient input is an experimental feature I added to this furnace, and since this is a "furnace" prototype, the recipe is auto-selected, so it selects whatever recipe it can find with the fluid you put into it using the sorting orders of all available recipes in the "smelting" and "chemistry" categories (and maybe a few others. ;) ). For most oils this turns out to be solid fuel, especially in the early game.