I only have limited experience, but..I will give you my thoughts....
Soft metals tend to smear. In small cuts you can even see this if the cutter is running backwards!!!! But don't. So the key is to keep the metal from sticking to the tool. My approach to this is to try to keep the tool cutting surface completely covered with the lubricant.... and cool. So I try to feed at a low enought rate that I can keep as much lubricant on the tool and work surface as possible to prevent the soft material from sticking to the tool. Carbon at the interface is good. Think of the oils as slick graphite. Oxide free rubbing metals tend to bond to each other, sticking.
When oils get hot at the cutting edge they chemical break down into more reactive chemicals. However, some oils have higher temperature tolerance than others. Animal fat is not particularly good at this.
In cooking the highest temperature oils last longer ..such as some seed oils. We cook with sunflower seed oil , etc because you can use higher temperatures before they breakdown.
I suppose the same is true with car motor oil. We change them because they wear out, breakdown, and get dirty from metal particles. The synthetic motor oils last much much longer, especially if you can filter out the metal particles. I've never tried them as a cutting lubricant, but maybe I should!
You will see what I am saying if you try to have an Al rail sliding on Al. It sticks and pulls pieces out. It's called spalling. I say oxide free, because Al oxide will slide ok until it wears off and gets rough.
So experiment a bit with you feeds and
speeds with the lubricant. So slow is fine if the cuts are not deep. But always feed fast enough that cutting is happening and avoid rubbing. Roughly I try to always cut atleast 0.005" per cutting edge in the feed process.
Good luck.
By the way, the highest breakdown temperature, tough, lubricant I know of is a Perfluoropolyether lubricant, but this too expensive for us to use here. We used to dip coat a very, very thin coating of this on the hard disk surfaces. One brand name was Fomblin® PFPE. A few ounces of this stuff undiluted used to cost us several hundred dollars. The solvent for diluting it was ether, but then that was band and 3M came up with another that was also very expensive, $500/gal.
