I have been detailing for over 12 years....I read everyone's post...here are my quick tips:
my preferred "over the counter" clay bars are 'clay magic' or 'mothers'
Use soap and water for the lubricant. You do not need to buy specific lubricant for the clay bar....it's a sales scheme.
Wash the car, then clay it, wash it again....then dry it and wax it.
The purpose of the clay is to remove surface contaminants
and airborn "fallout"
You will know when a section is done cause it will feel very smooth.
Clay does not remove scratches.
Do not use circular motions. Go "with the airflow" (that's my technique)
If anyone wants to talk further....feel free to Message me.
If you are an online shopper, check out autogeek.com
(I have no affiliation to them)