Back in the Dark Ages, when AMSOIL first introduced their 20w-50 as a racing oil, there was some red dye added to distinguish it.
Some years later, after a reformulation, the red dye was left out. Screams from the field, red dye was again added. Go figure!
At this point, think the red dye is only in the Series 2000 20W-50, not the regular (at least that's the way it is in the empties on my shelf).