Something I learned fast when I was a machinist trainee, was that just touching the surface left hand oils on the steel and the file would have a much different cut until the oil was either cleaned off, or cut away. I could actually feel the file slide across the surface instead of cutting it. This is with a new sharp file too. Any oils do make a difference in how a file will cut. Good point to bring up Cruiser.
That Moly trick is one I plan on trying. I brought my tube of engine assembly lube (moly) in from the shop just so I can try out that embedded trick.