k2skier

Yes to both. Multi-weight oils, and too much oil, (even a little) will do this. It doesn't take much of a slope, (15 degrees?) to make the oil come out the breather.

If it only does this on a steep slope, you are using it on a slope steeper than B&S recommends, it will damage the engine.

Try different angle, or directions to see if you can mow without it smoking.