Here's what the owner's manual says about it.
"The HC40 powerhead is an all position engine. The powerhead can be laid on any side for short periods of time. Do not store the unit up-side down. Store the unit with the spark plug facing up or the unit upright in the operating position when storing for long periods of time. Oil can migrate over time. Some oil leakage can occur over time. Check the oil level before each use."