I came across a 550 ton York YT. chiller and I suspect It has a high boiling residue %.(excessive oil) I have sent the refrigerant sample in for analysis. Comparing it with the other identical machine in the room it is quite visable. The oil in the sump is low and and doesn't seem to be coming back. A total of 5 gal has been added and while running the level is below the glass. Had the machine running and took readings. Chiller carryover is occuring. A 5 deg. dis. superheat and 5 deg approach at 100% Level in refrigerant s/g was not present. The cooler gpm is nowhere near design and has been like that for years.(design=8 psid actual 25 psid) Attempted to achieve design but insufficient flow throughtout caused problems therefore had to switch both chilled water pumps to run. Machine seems to run under light load conditions at times. What may have caused the excess oil in the refrigerant and any suggestions how to correct it and prevent it from happening again.