Checking on bicycle-engines. com the engines are shipped dry, no oil in them.
The engine requires 250ml of 30wt oil.
250ml = 8.453505639725oz.
Here's a link to the converter I used:
Convert ml to oz - Conversion of Measurement Units
Remember, 8.4oz is plumb full. 4oz is only 1/2 of plumb full, but it is still enough to not harm it if if ran for short periods of time.
Here's a link to the PDF manual. You can read it for your self if you want to.
If you actually squeezed 20 oz on oil in it, that is 2.5 times too much!