Proper Hub Oil Level

Hey guys I'm back with another Newbie question haha.
I was looking at my trailer and to me it looks like at least 2 of the 4 hubs are low on oil. I've included pictures of each hub in the post.
I'm seeing a lot of conflicting information on the internet about what it should be filled to. Some say the fill line, others say fill it to the point where if you stick your finger in, your finger tip gets wet. Lastly some say fill it right up the the plug hole like you would if you were filling a differential.
The trailer was an auction buy so I don't know the history. I don't see any seals leaking and inside the wheels by the drums look dry, no signs of oil leaks. I have a old 1983 Penski Dry Bulk trailer.
Thanks for any insight, this forum has been great.

 

When filling to to "full" line would you consider it properly filled in Hub 3 or is Hub 1 at a better full level?
Hub 3 almost looks like it is above the full line and overfilled.
I know the sight glasses are hard to see, sorry about that.

 

I just went out and dipped my finger in hub 3. It feels like good gear oil. I is a lot cleaner/newer looking gear oil than is in any of my oil hubs. Also is my newest looking sight glass so it makes me think the previous owner may have changed that one out not long ago or done some service and accidentally over filled it. Should I run it at that level or try to bring it down a little? A flush and fill would allow me to adjust the level.......