My old boss used to call stuff like that "grandma speeds" and then one day he crashed a face mill into a vise running hot feeds, so there's a middle ground somewhere. I ran a 3 flute on 6061 for years and settled around .008 to .012 chipload depending on the radial depth, with the spindle screaming at 8000 rpm. The finish got worse when I went too light because the tool just rubbed instead of cutting. Maybe bump it in small steps on the good parts and watch your spindle load meter.

Does he mean sulfur from the pipe itself? I ran into that on some foreign sch 80 years back and just thought my machine was acting up. Old timers really do know stuff you can't learn from a chart.

I read a thing a while back about how a bunch of tech writers in like 2005 swore podcasts were a fad that would never catch on because who wants to listen to random people talk. My uncle still gets reminded of that one at every BBQ, and he gets so mad about it. There was also that guy who ran a whole site saying the iPhone would flop because nobody wants a touchscreen with no buttons. Funny how that one aged.