HP doesn't even label how much ink is in their cartridges. I've had my HP Officejet Pro 8620 for 8 years. I'll bet there's less and less ink in the cartridge each year.
This is the right set of cartridges for my printer. It does bother me that all have to be replaced at the same time. I print in black and white so why do I have to replace all the cartridges.