You don't say if this is a single cyl. or a twin.
About the only other thing besides 1. just being worn out and 2. the crankcase breather that would cause it to blow oil is a blown head gasket.
On Briggs single cylinders, the head gaskets tend to blow into the pushrod gallery, and vent into the crankcase. You can't tell there is a problem from the outside and the unit usually will still run.
Does this unit have a history of backfiring when the switch is cut off? That can cause a blown head gasket. If so, best to keep the throttle up and just turn the switch off.