The lifter's filled with
oil...or....someone has probably shimmed the spring internally so that it couldn't pump up, which would jive with the zero preload statement that you made.
As for valve springs, do you have triple springs or double springs with dampers? Some AFR heads are set up for a solid roller and some solid roller springs could be too much for a hydraulic roller lifter/cam.
I normally run about 150-160 lbs seat pressure and 375-385 open pressure for my more aggressive hydraulic rollers. If you need help with selecting a spring, call me, or one of the other venders and we can help you. A cam manufacturer won't know exactly what to give you, since they don't know the exact install height of your current setup.
Normal lifter preload is 1/2-3/4 turn after zero lash. That's where I would stick it...