I generally use two types of coolant/lube
Dark sulpherized pipe cutting fluid which is dripped onto the job. This is for drilling and tapping, and some other general usage.
I also have flood coolant set up on both the lathe and the mill. When I first set up the mill, I was noticing that I was burning up cutters running them dry, and when I attacked stainless steel it was really quick. I set up the flood coolant with Semi Synthetic coolant and found that the life of my cutters gratly increased. so did the mess in my shop.
The lathe had always been a drip on approach or dry, but when I started doing parting off, knurling and threading more often, I set up the same semi synthetic and found likewise I had better results and was not having to juggle a can of fluid at the same time as everything else.
Remember that if you use carbide cutters, they have to be either run dry, or constantly cooled. If they are intermitantly cooled they can and will crack.
Walter