How to Use a PCIe 3.0 Card in a PCIe 2.0 Slot - GPU Republic
This compatibility also cuts across the versions of PCIe, so a PCIe 3.0 device can work in a PCIe 2.0 slot and vice versa.
Will PCIe 3.0 x4 10Gbe NIC Work in PCIe 3.0x1 Bandwidth?
I tested PCIe 3.0 x4 10Gbe NIC in a PCIe 3.0 x1 slot, and here's my thoughts and results. ... Therefore, can you use a PCIe 3.0 x4 10Gbe NIC in a PCIe 3.0 x1 slot.
Everything You Need to Know About PCIe 4.0 - trentonsystems.com
The PCIe standard is backward compatible, so an older PCIe card operating on PCIe 2.0 standard will fit in a PCIe 3.0 slot.