That seal is probably too big to fall directly down an oil passage down into the sump unless it was curled over. No worries, oil that's already free in the valve cover must fall and be filtered again before it is pressurized and used to lubricate. If it's just sloshing around, it won't cause any damage. It'll probably end up in pieces once broken down and be stuck to the oil pickup screen.
If you want, you can pull the valve cover and find it before it goes anywhere else.
__________________
'10 PW 370Z 7AT - wife's car - Project Raspberry Cheesecake
'08 Evo X - built motor+trans | BB-X @ 32 psi | CH3OH | self-built+tuned - ??? WHP
'01 Galant V6 - built motor | GT35R | self-built+tuned - 550+ WHP (retired) |