It varies. Some dealers will use appropriate oil, but others will cut corners. When I went for one of my "free" oil changes, I asked the dealer what oil they use. She said it is bulk Toyota 0W-20 made by Exxon, which was true as best I can tell. It shows up on the receipt as a Toyota part number.
A friend in Virginia bought a Subaru from a dealer that advertised free oil changes "for life" with purchase of the car. I wonder what they really use. He doesn't know that, or even how many miles they add to the current odometer reading when they put on the sticker telling him when to come back. So far (> 105k miles), no problems with the engine.