As I understand it, grey pearlitic cast iron has a high graphite content, and as a consequence it is self lubricating. That is why it is popular for use in engine blocks and cylinder liners. Of course, this explains why you don't NEED coolant/lubrication when machining it, but it doesn't explain WHY you shouldn't use lubrication/coolant when machining it. I just googled it, and the Sandvic website confirmed what I thought about not needing lubricant, but it didn't say anything about not being supposed to use lubricant. On a deep plunge cut like the fins require, I would think that the use of lubricant would help flush out chips and keep the tool from grabbing and/or binding, but I don't know for sure.