1974 Honda st70 what is the small drain plug for

What is the small drain plug on the bottom of the engine and what oil goes in it thanks.

Hi, the small nut on the bottom of the engine has a spring and plunger behind it which is the camchain tensioner and pressure spring, the other nut is the oil drain plug for draining the engine oil out of

I use a oil called ROCK OIL 10W40 mineral in my ST70