Most of my work is on steel. My favorite cutting oil is Vipers Venom. It is a high sulphur oil for cutting steels including stainless. It excels at precision work and has a unique slinging property that facilitates two or three passes with one application. The problem with it is supply. It used to be available from Gary Eakin in BC, but he has quit carrying it. If you can get to a Grizzly Outlet in the US, they carry it in quart bottles and in a 5 gallon pail. I'm going to try getting a pail shipped to me but I'm not optimistic. If I am successful, I'll happily share for the cost of shipping. If you can't get venom, I have tried regular tap magic
I use WD-40 on aluminium. You can get bulk wd-40 at princess auto. If you prefer a dedicated cutting oil, Tap Magic for Aluminium prevents tip welding and is pretty good stuff!
I usually cut brass and cast iron dry. But kerosene works well if you find you need something. Don't use sulphurized oil on brass - it will discolour it.
If you want one oil for everything, try Tap Magic EP-Xtra. You can buy it bulk, dropper, and even spray cans. It's new for me, but Tap Magic is an old company with a good reputation.
The way I look at cutting oils is different from what most guys will say. The "cutting" process on metals isn't really cutting. On a microscopic level, metal cutting is more like gouging. The edge of the cutter compresses metal ahead of it and the metal separates at the strain lines ahead of the edge. The process can be controlled and that's what edge shape, speed, and feed are all about.
Cutting oils cannot change the way the stress and strain play out in the metal. But they can change the way the chip absorbs heat, and the way it sticks to the cutting edge or slides along the cutter, and the way the chip contaminates the cut ahead of it. In my opinion, the following videos should be required viewing for all machinists. After you watch them, you will probably think differently about cutting oils.
Last but not least, there are a million different points of view on this subject. Mine is just one of them.
Enough for now. I'm gunna go and see if Grizzly will ship me some vipers venom!