You'll have to run much richer than that to have problems with washdown. You'll be fine, and the worst thing that will happen is the plugs might foul. If you carry a wire toothbrush with you you can just give the plugs a quick cleaning on the way home if you need to, but I seriously doubt plug fouling will be a problem.

Don't skimp on the oil change intervals though, because you might end up with a little more fuel than normal in the oil. I'm sure you're planning on changing the oil when you get it home anyway though, which is the first thing I do with a new car.

Unless you're using a wideband O2 sensor I wouldn't even bother trusting what the gauge says.