It's worth noting the oil revenue was largely supposed to make up for the loss of funding from the Saudis and the Gulf states, because as international attention ramped up, ISIS started receiving less and less funding from private individuals.
To go back to what I was pointing to before, which is relevant to the question of recruitment, it is that these were multi-generational investments into radicalizing the Sunni world, across the Sunni world. Basically, in more immediate terms, yes, the recruitment is largely occurring online and I think there's really not much way of stopping that.
I would also point out that in addition to what Professor Momani pointed to, there are a lot of small refineries in Syria that are under ISIS control, and those have been targeted by U.S. air strikes I understand. I don't know what the capacity from those is, but yes, it is definitely worth mentioning.
This isn't to say that they don't have control of oil wells, but without refining capacity they're not going to be able to sell oil that anyone wants to buy. That's a little bit too sweeping, but generally speaking this is definitely going to lower revenues. As I said, it's likely to result in more pressure on minorities and also on foreigners in ISIS-controlled areas.