I assumed he could possess it now because of the fuel inside it. He couldn't do anything at first because there was no path to the fuel inside. It was a completely sealed container. Once he ruptured it's elbow it created an opening, which allowed him to infect it. As for why he could potentially possess it, it would maybe be because fuel is technically organic matter (assuming it's oil we're talking about). Typical oil, the black sludge most people think of, is made from the remains of long dead animals and plants. It's carbon. So, theoretically, he could indeed influence anything that runs off the stuff.
I'm honestly still doubtful of his ability to posses it in it's entirety since he'd only be controlling the "blood" of the construct and not it's mechanical nervous system or robotic brain, but maybe he will develop an adaption that allows him to take full control like someone else in the comments suggested. Y'know, the whole computer virus idea.
Edit: For context, it was the comment made by karappo that mentioned the computer virus idea on the previous comments page.