There are 231 cubic inches in a gallon of anything. If you are confident of your measurements it is simply length x width x depth for the total cubic inches displaced by the pan dimensions. Divide by 231 to get gallons and multiply by 4 to get quarts. Add an additional quart for a remote filter.
Be careful when you measure the pan depth dimension. This dimension will determine the
oil level in the pan when the engine is off. The quick math (based on your measurements) without regard for the presence or absence of a windage tray looks like this 18 x 12 x 6 = 1296 cubic inches. 1296 / 231= 5.61 gallons. Four quarts per gallon works out to 22 quarts.
22 quarts of
oil is huge! My suspicion is that you measured to the top of the pan's gasket surface. If your
oil pan has a windage tray (which is to your advantage) you need to measure to the underside of the windage tray. If you take the pan off to get the windage tray measurement then you might as well just measure how many quarts you have to pour in to bring the oil level to the windage tray.
For those doing a new build and in a first start scenario it will take approximately 1 more quart to wet the inside of the engine. So be sure to take that into consideration.
The obvious danger to an inadequate oil volume (in the pan) is the potential to draw air instead of oil into the system. The presence of air in the oil system, especially under load, will do permanent damage to engine bearings and bearing surfaces.
Oil not only provides lubrication but also cooling for the engine. Reduced oil capacity reduces the cooling which is almost as bad as drawing air up an exposed oil pickup.
Your pan looks like an Aviad. If it is, the short way home is to call Aviad and ask what capacity it is. If it is not an Aviad then you are still on the Easter egg hunt for oil pan capacity.
Regardless of how you finally make your oil pan capacity determination do a preliminary check with the engine off and then do a second check with the engine running, up to temperature and idling in neutral. If the level has fallen add sufficient oil to bring it back to the the level on your dipstick that corresponds to your windage tray level.
Record the total system capacity wherever you keep track of you engine stats.
Ed