I've been using a heavy cutting oil on steel. It makes a hell of a mess and the work (4130) gets pretty hot, quickly. My lathe has a coolant pump system but I've never used it. Guys have talked about the fluid growing stuff and it's short life. I want to give it a try. I'm asking for recommendations. I usually do all roughing with carbide inserts and switch to HSS tools for the finish. As usual for a home shop, I also use aluminum quite a lot also.