It's the same sealing washer used on the head cover bolts and the three clutch cover bolts.  OEM part number is 09168-06023.

OCH, that McMaster & Carr gasket is solid rubber.  You can't stress the fastener properly because the rubber will deflect.  The OEM gasket is a neoprene insert surrounded by a metal ring.  When you tighten the fastener the rubber deflects until the fastener contacts the metal ring.  Then the fastening system is metal-to-metal and you can properly stretch the bolt.

Thanks, DBM!