Like you, I have never used anything other than Epson inks for colour. What I was referring to was the use of 3rd party black insets, specifically Cone Piezography inks. On the subject of drift, I can not say categorically what causes it but I can say that linearity drifts over time. It may be the ink, the paper, the printer, humidity, temperature or whatever combination of them but linearity does shift. I measure with an i1Pro2 on an i1io2 in 2 pass spot mode on a 21x4 target, and then map it in Excel. It may not be much but I'm sufficiently anal to like straight line linearity.