Oval carabiners are the best choice for a crab where any pulley is to be used. This is because the pulley will sit centrally rather than being forced into a corner as would a regular D shaped crab. This means that they stay in a correctly loaded an equalised position. The symmetrical shape means that a pulley can be used on either side, keeping things simple.
Other points to mention are the Keylock nose, to prevent snagging on ropes, gear etc. Also a Red visual indicator shows when the carabiner is unlocked.
If you are looking for a crab and pulley for light load situations, consider the Petzl Ultralegere Pulley Wheel as an additional product that works perfectly on this OK Oval carabiner.