Yamaha PG1 Engine Oil Changing Tip (actually stated in the manual book)

If you're changing the engine oil by yourself or if you're planning to send it to a workshop, make sure after draining the engine oil you open this bleed bolt and start the engine until a little oil flows out. This is to remove trapped air pockets from the engine oil circulation system. Many people overlook this. Workshops usually don’t do this step. If you start the engine, you will hear the sound of air bubbles coming out from this hole. You can refer to the manual where it is actually written there.

In order to do this you'll need socket no 12 or wrench with that number. It's quite tight & you need to use great force to unscrew it but once loosen start the engine & let some oil come out from the thread. Once done don't forget to tighten the nut back but make sure not to tighten it too much as it may wear off the thread.