What is the reason why the throttle of the old Mercedes-Benz 300S has become so heavy?

The throttle of the old 300S is a mechanical throttle, which is driven by a throttle cable, similar to the brake cable of a bicycle. So it is normal to wear out and lack oil after a long time. Of course it's heavy to step on the accelerator at this time. It is suggested to change a throttle line to ensure the problem is solved.

Off topic, let me tell you something about the electronic throttle. It works like this. The accelerator pedal you step on controls a potentiometer (to put it mildly, a sensor), not directly controls the accelerator. The potentiometer receives the starting stroke, that is, the current, and transmits it to the control circuit (also called computer). Then the control circuit controls the electric pickup on the accelerator to drive the accelerator to open, which is the size of the accelerator pedal you step on.

His shortcoming is that the accelerator response is slow, and his advantage is that the car is still very light when it drives to the scrapped accelerator pedal.