No, you do not need to use genuine parts. That is a myth. Aftermarket parts are just as good if not better than genuine parts. However in saying that, stay away from anything that comes that looks cheap.[https://www.ebay.com.au/itm/Subaru-Valve-Cover-Gaskets-Kit-Impreza-Forester-Outback-Legacy-SOHC-99-05-OEM-/190638466610|This example] is something I would buy, due to the fact that the money is OK and it comes with all the necessary parts.