What is different/special about recirculation oil VS hydraulic oil? To the best of my knowledge my lathe does not have a pump, I believe everything is 'splash' oiled. would that make much difference?
Circulating oil will put a higher emphasis on high temperature oxidation, stay in grade over a higher temperature range, has better heat transfer properties, Along with that, it's (kinda) geared towards higher dollar equipment and machinery, and is easier to find in a higher quality overall product.
Are the gear boxes in these machine tools, that survived the absolute best oils available thiryt, fifty, seventy years ago, really testing the abilities of a modern oil? I'm going with "no". but I still feel better knowing that I've got a premium, trustworthy, "known commodity" in my stuff.
In a controlled envrionment, in low hour machines, in hobby shops, and generally not being used at a low duty cycle and generally low loads for the design intent... These oils are not "that" far removed from each other.
FWIW, good luck finding an actual "line" that the oil must cross to move from one category to the other.
