it will rev when i stop, or drop so low once i stop i think the engine is about to die, etc
That’s not about the transmission. And oil change won’t do a thing. A transmission fluid change won’t do a thing either - although I’m betting you’re long overdue.
Ask around among people that you’ve met there. Find a trustworthy locally owned auto shop. Do not go to national chain shops and do not go to dealerships. Just a small, local auto shop - the kind where the guy’s name on the sign is the same one you talk to about your car.
Take it there. Have them get you back on track for taking care of this thing.