aFe Power differential covers come with the option to run a oil level sight glass to visually see how much oil your differential cover has at all times. Factory and most aftermarket differential covers do not allow for this option which leaves you guessing on your fluid level. Dont be caught with low oil on your differential, or your up for a costly repair aFe Power has got you covered This is a great upgrade for all aFe Power Raw or Street Series differential covers.