Almost all motorcycles and scooters in Thailand, are equipped with hydraulic front forks. Most motorcycle mechanics do not realize that the only function of the internal fork spring is to hold up the motorcycle.
A physical property of air is that it will compress. Therefore air inside the fork will compress.
A physical property of oil is that it cannot compress. The fork oil's function is to transfer through orifices in the hydraulic fork internal dampening rods, which are most of the time below the fork springs, and regulate compression and extension of the forks. The hydraulic fork oil's viscosity (its rate of flow), orifice hole size, and number of orifices combine to produce dampening rates for different functional types of hydraulic front forks.
Inadequate oil in the forks screws up the carefully designed balance. And can cause many undesired, and some even dangerous, side effects.