Onan 4KY Consuming Oil After Change, Breather Tube/Crankcase

I've changed a number of older engines to Mobil 1 oils over the years without leakage or oil consumption problems (and if it was leaking, you should be able to see it).

I'm not familiar with this particular engine, but a lot of small, air-cooled engines have rubber flapper type valves to control vapor from the crankshaft going to the intake. If they fail, lots of excess vapor and oil from the crankcase can be routed to the intake.