OP, george wilson, I'm glad to hear about this update on the "hecho en Mexico" Nicholson files! I use lots of diemaker or Swiss pattern files in my hobby gunsmithing work. The last set I bought were so soft that they were useless! I bought them from a vendor at a local gun show and haven't seen him since!
My files are treated with great respect, however I was taught to never oil a file. The diemaker that I served my apprenticeship under, in the early 70s, said that a light surface rust was preferable to oil. In fact he actually treated his dull files by leaving them in a coffee can full of water out behind the shop until they "rusted sharp" again! Anyone else ever heard of this?
Another trick he taught me that I use to this day was: In his toolbox, he kept a large piece of chalk (the kind that carpenters used to recharge their string lines). He would always pass his file across the chalk to load up the teeth, before making his cut, after that, he would recharge as necessary with the chalk. The chalk would then keep chips from loading the teeth. I have never tried resharpening a file using rust, but this chalk trick really does work.