A sight glass is installed below the fill port scupper ____ at a level approximately 3 quarts (3 liters) below full. There are two conditions when the tank is full: - When the sight glass has dark color, the oil tank is between full to 3 quarts (3 liters) below full. - When the sight glass is clear (does not have a dark color), the oil tank is at least 3 quarts (3 liters) below full..