(I could be wrong) but never saw (40+yrs) any black brass UNTIL they came out w/ E-gas; I would more suspect the problems/ corrosion you saw came from ethanol/ water. I also had NEVER seen chalky white deposits until E-gas, and they were in my Tecumseh carburetor (black plastic w/ stainless); that was when I switched to pure gas ONLY for my small engines.
(WHAT I WANT TO SEE) on a label is "Synthetic" and the rating of "FD" & ""LEG-D" for JASO and ISO standards. Echo Powerblend meets this, and (I assume at this point) that other (Name Brand) synthetics do also. Echo 5-yr consumer warranty and experiencing good results is what convinced me.
RESULTS I WANT to see are: 1) No carbon buildup; 2) No varnish; 3) No cylinder or piston skirt scuffing/ scratching; 4) No locked down/ sticky rings.
I would NOT be afraid of Lucas or Redline, Stihl, Husqvarna, nor any other major brand, but would only only use Non-synthetic if no choice in a pinch. I actually am surprised that Stihl still sells BOTH?