I feed sheets in my 8400 (and 8300 before that) all the time. Line them up with the orange lines as suggested and maybe 1 in a 100 has to be re-loaded. If the paper skews during loading, try guiding it in with a finger on each side. I've successfully load sheets as small as 7.9"x7.9". If you are losing more than .2" of unprintable paper on the leading edge, go to Paper Menu>Paper Detail Settings>Media type for your paper>Near end sheet margin and choose 5mm instead of 20mm.
Look at the settings available in "Paper Detail Settings." A lot of problems can be fixed from within this folder.
Wow. Glad I almost never print on sheets. Sheet loading the ipf8300 is a royal pain. Trying to load 11" wide sheet. Won't take it. Anyone know the minimum sheet size on this printer? I'm guessing it has to be bigger than 11" wide.
UPDATE: It's an 11x28 sheet. Even tried turning it horizontal and the printer won't take it. Says "Paper size not detected." Guess I'll stick to my z3100 when I need to print on sheets, which is thankfully not too often!