Genuine Shin-Etsu silicone grease. 

For best results, clean your joystick regularly to avoid wear of actuator and other parts. Do not mix with any other grease or lubricants.   

***Just to be clear, you are not getting a whole tube of the silicone grease....you will get a small sample size about the size of 2 nickels.....which will last you at least 20 applications if not more.