Who:

The dents are caused by excessive lube these dents are called hydraulic dents. One cannot compress a liquid, so the lube gets trapped between the inside die surface, and the case when sizing. The die is stronger than the brass, so the brass dents because the lube has to go somewhere which causes a dent.

You just want to very very lightly coat the case.

What kind of lube is a presonal preference. Many people like the spray better. I have never used the spray just the lube pad. One guy on this forum uses some kind of wax he says its the best. Good luck.