How to set 19.5Hz PWM frequency?

Hehehehe!
Nick what i want to do is control a solenoid from a car! People have reported the 19.5Hz frequency to be the correct one to control it!
I haven't measured the output of the car's ECU but if everyone says that the frequency is 19.5Hz then this is it!

Any code examples to get an idea? Or a guide?

If i change the pwm frequency in this sketch, will i have to change it if i build a new sketch? Do i have to set it back to default?

Thanx guys