Just some normal hand cream.The brand is Nivea, I don't know if you can buy that in America. That shouldn't matter, though.
When assembling the trackball, mind the location of the rollers. It is possible to position them slightly off, which results in cranky movement. I had to take my trackball apart again to correct that.
Which reminds me : Do a test BEFORE you put it back into your CP. And before you screw the CP back into the cab. I didn't.