Can't find the correct drain plug adapter for my lower unit

I have a 1996 NS40C Nissan Motor and I need to fill it up with gear oil. Everything I have seen online says that the drain/fill plug for the lower unit is 8mm. That is indeed the size of the upper (vent) screw. However the lower one is definitely smaller, I'm guess 6 mm. I can't find an adapter for my oil pump that size and I can't manage to get oil in there just trying to hold a tube tight to the opening.

Does anyone have any experience with this?

Should I just fill it from the top, let it sit, fill it again, let it sit, etc to let the air out?

I'd appreciate any advice.

Stop.
You have the wrong screw, probably the cam rod stopper. The fill and drain plugs are identical.

Ah, the blessed internet. Thank you so much Paul! I just didn't look low enough. It is even stamped "oil drain" on the the engine itself! I feel like an idiot. Thanks Again.