That's probably it. The manual I looked at online at Briggs and Stratton's web site give two amounts for two different engines. One was 36 oz., the other 48. That's quite a bit of difference. If it's too full, it's got to go somewhere, and out the breather is where it's going to go. Just add a little at a time and re-check the dip stick as you do until you reach the full line.