Yes Clive, I was referring to the oxygen present in the water (or developer stock solution), not the one that dissolves when left in an open container.

I also mix my developer the same way as you do.

I only quoted PE to highlight that the problem is real, on the other hand ID-11/D76 is pretty robust and my system is not that precise that I could point with great certainty any variations in negative density to the aging of my developer.