The syngas refining recipe to make Heavy Oil primary is unlocked by flammables(Logistics), but you can't make syngas until Organic Chemistry(Chemical), which is a whole science pack up. Not sure if intentional or not, but it's kinda useless being unlocked there, from my experience.

You should be able to make syngas once you research flammables using the gasifier. It takes steam and any chemical fuel.

Is that how it works? Okay then, that makes a bit more sense then.

Yep, just stick the steam and some wood in there, or whatever, and you should be in business! If you don't have the steam cracking setting turned on, you might need to use water instead.