What will cause a delay when shifting an automatic transmission from neutral to drive or reverse?

Low oil level in an automatic transmission can cause a delay when shifting from neutral to drive or reverse because the transmission relies on hydraulic pressure created by the transmission fluid to operate its internal components. When the oil level is low, there may not be enough fluid to generate the necessary hydraulic pressure, leading to slower engagement between gears. Insufficient lubrication can also hinder the movement of the valve body and other components responsible for transitioning through the gears, resulting in a noticeable delay.

While high oil level, worn torque converter seals, and worn low-speed clutch packs can cause other types of issues within the transmission, they do not primarily affect the hydraulic pressure needed for immediate gear engagement in the same way low oil level does. Therefore, low oil level is the most direct reason for experiencing a delay when shifting in an automatic transmission.