There is no oil that is so bad that it disappears****! Your mechanic (or he/she who thinks they are) obviously either forgot to put the new oil in the engine, or neglected to replace the bolt on the oil pan, or replaced it loosely and it came off, letting all the oil pour out.
That happened to a friend of mine years ago and she got a new engine out of it--free from the shop that did it (or, rather, failed to do it right). Get your receipt and go back to that shop/garage ASAP and tell them what happened. If they do not ofer to repair any and ll damage, explain to them that you already consulted your cousin about this and that cousin also happens to be a lawyer. Your lawyer/cousin told you that you should sue them if they refuse to do the necessary work to return your vehicle to the condition it was in when you first brought it to them. If they still refuse, then you will definitely have to see a lawyer.
I don't know what book value on your 99 GMC is, but if it's under about $5,000 then you might want to pursue it in small claims court. That's if you don't have the bucks to put out for a lawyer. However, keep in min that, when it comes to law suits, most lawyers will forgo payment until the case has been settled. They then deduct their percentage from the settlement amount. And don't forget to mention all the peripheral things it affected in your life--the stress, the loss of work due to inability to commute there and back. The monetary problems you had from unpaid bills because you lost work time. You'd have to have some type of proof for a lot of those, but the 'emotional distress' is not usually a have-to-prove thing unless it's long-term. And don't be shy. Tell them how your girlfriend of five years almost left you (or DID leave you) because of this and how you were constantly fighting because you couldn't go to events, or make it to appointments, or even food shopping (as long as you don 't live in an apartment on top of a supermarket).
Good luck.That's a really crappy thing to happen. Lucky you weren't in some remote place or wilderness at the time. I once drove a car at 60-plus miles per hour for 75 miles into NY City with zero lug nuts on one of my wheels after a tire rotation. The idiot who thought he should work with cars, forgot to replace the lugs on the last tire. When I went back to tell them and the manager looked it up on the computer he saw who did the work. Apparently that guy had done similar things before and, needless to say, he lost his job. I felt bad but I could very well have lost my life! 60 or 65 mph for 75 miles with one tire un-attached is not recommended by most safety authorities, if you catch my drift. I didn't sue but I probably should have. You could have lost your life too. What if the engine had seized while you were merging onto a highway? The cars behind you would all have been accelerating and would have rear-ended you.That might have been serious or fatal!
If you start to feel badly when talking to them, think about that. Hard! Think about all the things and all the people that would have been affected by it if it did happen, because it could have happened.
Again, good luck.