What does putting oil in a car, without draining the old oil do?

So…my car putters to a stop, I push it to the curb, and with no money for gas, find a full bottle of oil in the hood under the ignition thingy. I decide to pour it in, and because of that the car starts, and we manage to borrow some money from a friend and make it to a gas station which happened to be the problem from the start. Now I have two problems. A broken gas gauge which is why I put the oil in, and a car with some unknown oil in it…

Does anyone know what this means for my car?
I’m planning on taking it to Les Shwab tomorrow, but…I’m just wondering if anything knows anything about this.

As far as my car, it’s an 84 Nissan 300zx. And the oil was just…I dunno, green bottle.