I don't add oil when I make tahini, but what I get isn't as "runny" as jarred tahini, so if you wanted a more liquid-y consistency, you could add a bit of light sesame oil. Doing that would likely make it smoother, too. If I was going to do that, what I would do is grind the seeds first, then add oil a little at a time until I liked the way it looked. I don't think it would take much.