all officially supported cpu's are listed in the HP z820 quickspecs, note that this quickspecs has been updated several times during the z820's production run ypur board is a v1 55xx only board
early z820 boards did not support v2 cpu's (bootblock 12/28/2011, 55xx Nehalem only) AS#619562-001
later board revisions do (bootblock 3/6/2013 Westmere 56xx and 55xx) AS#618266-003
https://www8.hp.com/h20195/v2/getpdf.aspx/c04111526.pdf
last,..... your not looking that hard, as this has been covered hundreds of time on this forum (use the SEARCH feature!!)
and google will show hundreds of links also