PCIe x16 GPU card into a PCIe x8 slot?

Is PCIe x16 the standardcommon interfaceport on a modern pcie slot x16 PCIe slot will run at the lowest denomination speed The most common PCIe slot currently in use are PCIe x16 slots, PCIe x4 pciex16 slot configuration List of Motherboards with 4 PCI-Express x16 Slots: ASUS Pro WS TRX50-SAGE WIFI, GIGABYTE A620M C, GIGABYTE B650M C V2, ASUS Prime B760M-A AX, GIGABYTE B760M

pcie 30 in 40 slot Buy PCI Express X1 to X16 Low Profile Slot Extension Adapter