I have a 2012 chevy suburban. i noticed the inside of my drivers side tire has an oily substance on it. what do you think is leaking?

Without looking at it I cannot be a 100%, but it could very well be brake fluid from a bad brake line. But again I'm just guessing

Thanks for asking us. We do have a Service Center if you find you need repairs, but the only fluid typically behind the wheel is brake fluid from a problem with a line or from a caliper. This can be from age of a part or excessive wear if it gets too bad. A good place to go if you questions like this, is the Car Question Group which is part of our Under the Hood Show Facebook page, Join the group and there are people asking questions and answering questions for each other. Some great info, we call it Hoodies helping Hoodies. Don't wait on checking into that problem as a loss of hydraulic pressure is possible, if that is what it is. Speculation of course.