The oil is full when is just touches the upper hash marks on the dip stick. You have to unscrew the dip stick, wipe it clean but do not screw it back into the fill port.
You're supposed to line up the dip stick like you would if you were going to screw it back into the fill port (without actually screwing it back in) and then check the oil level. I have a GX390 on my new Honda EM6500SX Generator and pages 43 (Engine Oil Check) & 44 (Change Engine Oil) in the Servicing your Generator section provides a drawing showing exactly how to check and change the oil.
Here's a link to the manual. You may have to do a copy and paste: http://powerequipment.honda.com/pdf/manuals/00X31Z236000.pdf
This message was modified Sep 24, 2012 by FrankMA
Toro Wheel Horse 522xi GT, Honda HS928TA, Honda HS621AS, Honda HS520A, Toro CCR3000 (work in progress), Honda HS624WA (sold 08/23/2010), Stihl BR550 Backpack Blower, Stihl MS250, McCulloch MS1635, Honda EM6500SX Generator