The SOHC heads are designed so that the tappets are very well lubricated. This reduces friction and wear. All of that occurs on the top floor of the head, so to speak. On the next level down, is where the valve springs are seated, and the valve guide tops are. And that is where the valve stem seals are located as well. The design of the tappets is such that the valve stems and guides are not exposed to excessive amounts of stray oil. Any oil that does come down past the tappets easily flows down the return passages to the sump.
Because of this intelligent design, valve stem seals are only needed on the intake valve stems and guides.
Here we see #3 intake valve in; and its stem seal installed on its guide. #4 intake guide is ready for the valve to be inserted; and then for the valve stem seal to be installed.