Oh snap.
I thought you meant you used mixed gasoline in the mower. If that was the case, you wouldn't hurt anything by running a tank of that through it, although it would smoke more.
If you put two cycle oil in the crank case, though, you need to change the oil immediately and fill it with the correct oil.
Two cycle oil is a 20 weight oil or less, it is not suitable for a 4-cycle lawnmower.