Oil for onan 2500 lp

I've had both the Onan 4K and 5.5K units and never measured the oil while performing maintenance, just fill to the oil stick inlet. I know it is not very much oil with either unit, we always use full synthetic as it rarely takes more than a quart. Onan manuals are readily available online as I have downloaded them in the past, never had need to peruse through them, just like having manuals for all onboard equipment handy should issues arise.