I have read that you are not suppose to lubricate these. That being said I always used spray on gear lubricant on the JD650, this stuff really sticks. Now I use Fluid Film on just about everything. Fluid film does not work as well as WD40 for cleaning your hands but for everything else I'm good to go. The JD 2320 came to me with so much paint on the balls it was impossible to move them without using a rather large drift punch, loose is good on these.
The owners manual for the 4520 says that the JD SuperLube can be used on the ball joints of the 3 PT hitch. So I've been using that. The smallest possible squirt happens after each mowing, go through about 2 cans a year, cheap insurance.
I used to use 30 weight oil, but is seems like that make a black film and attracts dirt more than the SuperLube.
I have and use Fluid Film too. It's great for spraying on the sickle bar after I use that.
I suspect that like many lubrication questions: It's more "do you keep up with it" than it is "do you use the right magic stuff". I can also see where indoor vs. outdoor storage plays a hand in here too. For me, the "less than 30 seconds to hit all the ball joints" that comes from a spray can makes sure that I do it after each big use.