pci express - PCIe x8 NIC in PCIe x16 slot - Server Fault
According to this this diagram of the chipset, the x16 PCIe slot is connected differently than the other PCIe slots.
HP Z440 Tower Workstation | IT Creations - itcreations.com
Which PCIe slots support the HP Z Turbo Drive G2? ... For all HP Workstation platforms the tested and approved slots for the HP Z Turbo Drive G2 are as follows ...
HP Z8 Workstation | 4th Gen | HP® Store
HP Z8 G4 90 1450W Chassis 100V/20A. ... Slots: 2 PCIe x4; 3 PCIe x8; 4 PCIe x16.