The main thing is that you do not want to get too much oil into the crankcase... that would cause foaming and potential starvation to the bearings and also oxidation of the oil... taking its useful life to almost nothing...while you have no way of being warned of this while it is happening..

You put less than you know it needs, start the engine, stop the engine, let it sit a while, check the dipstick... then add a little at a time and check it again...