You know I was hoping the 2nd movie would continue on the 2012 story arc and wrap things up a little better. I should have known better. It wasn't an awful movie, but if they ever do another one i'd much rather them build off the show rather than make basically a 2 hour episode about an x file that's completely unrelated to anything that happened in the show.
The black oil stuff was the alien virus that took control of people and eventually turned them into super soldiers. That's how Interpreted it. The reason it took on the form of oil is because it was actually dormant for thousands of years and was only able to survive in underground petroleum fields. They explain all of that In season 8 I think and then again in the very last episode. The alien conspiracy plot is a bit tough to follow at times because it took so many plot turns. They give you one reason and then 2 seasons later they come up with a new explanation. So just when you think you've got a good handle on what's going the writers change it up on you.
The incest episode reminded me of "The Hills Have Eyes". Didn't like what happened to the sherriff but that was still a really good episode. They could have come back and done another episode on that family but I'm glad they didn't. Since they did just about everything they could to keep to themselves it makes sense that they weren't likely to resurface quickly.