I have always used Rigid dark because I could get it from work free. I worked as a material manager on several Industrial Electrical jobs. We would get sent Rigid cutting oil to the jobs, dark or clear depending on what was cheaper at the wholesaler.
From the jobs that I managed over the years, I noticed some jobs had a much higher rate or tool wear on our threading dies. These dies are used on the Rigid 300, 500, 1000 power threaders, sort of a lathe for threading Electrical rigid conduits and other pipes up to 2"id.
It turned out the jobs using the Nu-clear had the much higher wear rates. So no more Nu clear for us and no more for me!