Answered: Can I Put a PCIe X4 or X8 Card in an X16 Slot?
If you put this in a PCI-e 2.0 slot, the transfer speed will be 16Gbps, please note that. For full speed you must plug device into a PCI-e 3.0 slot.
Can I Use a PCIe 3.0 Card in a 2.0 Slot? - PC Guide 101
Slot Type Size (Pins) Common Use Mini PCI 100 or 124 Wireless cards Mini PCIe 52 SSD storage, wireless cards. ... Yes, PCIe is backward compatible, so you can use a PCIe 3.0 card in a PCIe 4.0 slot without issues.
What is PCI Express? Do PCIe lanes, slots, and versions matter?
While you can install any PCI Express card in any size PCI Express slot, you should match the type of card with the type of slot, for example by installing a PCI Express x8 card in a PCI Express x8 slot.