On advice from Lycoming, I filled my O-320 D1A full of oil for preservation since it will be quite awhile until I fly. It took 7 gallons! They advised using cheap automotive oil, which I did use. To completely fill I had to tip the engine down in front and fill thru one of the plugs near the top of the engine.
At maybe $4 per quart, or $16 per gallon for aviation oil, you may want to try something else.
If you keep it full for long, you'll notice it will leak.