Just to be sure and correct me if I'm wrong, but is it a standard amount that all 4 cycle push mower take exactly 20 oz. of oil?
20 oz. is probably most typical, but not in every case.
They've been making these things for 60+ years.
Robar: Use the old tried and proven method . Utilize your Engine Oil Dip Stick . As Bill has advised 20 oz. is normal but once your crankcase has been drained add 10 oz and check level with dip stick and only fill to just under full . Better a little low than over filled which will be deposited within you air filter and cause carbon deposits and worse .
My Lawn Boy 4 cycle with 6.5 hp has a dipstick, but for what ever reason, the oil level is just barely touching the bottom of the dipstick.....even after I added a good amount. I'll probably just drain it and refill to be safe.
Lawnboy oh Hell where is Orange when you need him . Yeah drain and be safe rather than sorry Dude ! 6.5 is the large block model may require additional crankcase oil .