Fuji vs kodak e surface
Like Penguin says, pro labs like us calibrate the digital processors every few days or when the emulsion number on the paper changes. This is why we buy in bulk. Our supplier provides us with matched emulsion numbers. Even though I have not seen any significant differences between two different emulsion numbers, they may be slightly different because of different materials used to manufacture the paper.
Typically, we calibrate our processors before a big print job of 5,000 or more prints. Or when we are printing several hundred 12x18 to 20x30 images.
Chemistry does not stay in our inventory long enough for it to be a concern. Our monthly order is usually gone within 3 weeks. To make sure chemistry residue does not stick to our paper processor racks or tanks, we rinse off the racks (2) each day every day those processors are producing prints. After a huge print job, we examine the racks to make sure nothing is out of order.
For most customers, I doubt they can tell the difference between Kodak and Fuji but we prefer Fuji. In the old says, I used to prefer Kodak just because they had the name and history. This is not the case anymore as Kodak is really downsized since the 1980s.