workshop compressor oil leak any advice pls

one of my compressors is slightly starting to leak oil,what is the part called & what does it do,is it rebuildable or does it have to be changed for a new one ,should there actually be oil in this part at all ? obviously I want to cure it but compressors are not my thing I just switch them on when I open the doors for trade. any advice pls. the first pic is the offending part & the others are the machine. also its 3 phase,250 ltr running 10 bar 150psi, year 1977 same as a atlas.i know its old but its strong & super smooth/quite ive worn out far newer compressors than this but it still keeps going like the day it was made. would rather save it than scrap it. thanks in advance