I use WD-40 pens, and just dab a little on the swash, between the blade grips, and along the head for the collective movement. It works OK, but can attract dirt, so I clean often with high quality electronics spray cleaner, and re-lube.

The best thing to use on the top plastic part of the rotor head to lube the collective movement is powdered graphite (most commonly used to lube locks). It does not attract dirt, and works a little better on plastic than light oil. Like I said, however, you can use light oil, but be prepared to clean it often due to it attracting dirt...