Proper Hub Oil Level

I work at a place that rotates a lot of older trailers through here. Some are sealed with synthetic grease. Some have hub caps like yours with the plugs siliconed in and synthetic or petroleum grease in them. Others are running oil. Maintenance records got lost in computer land a few years ago, they are buying used (refurbished) trailers and they do not want to pull the hubs on all these things.
It is a good idea to jack them up and be sure bearing adjustment is not loose. Those ones with grease (that are not the newer factory ones) I pull the plug and put synthetic oil in it until it gets to the bottom of the hole. That is done every time I see it until it gets to the full line or the seal starts leaking. Odd yes but I have not lost another axle. Put a pencil magnet in the bottom at every service to check for metal. Some hub cap plugs ( on the side ) have a magnet in them. Your can rub that off with your finger through the center hole.
Nothing wrong with a little over full if it is not blowing oil and or doing those other smart things. No a bad idea to pull the whole assembly when you are doing brakes.
Around 1986 IHC went to the smaller the 3782 3720 bearing set (which is common on many brands) and introduced the set back axle. They were having a lot of bearing problems. Because they carry more weight on the front empty or something else? They came out with a hub cap that the plug fits in very tightly and the file level is the bottom of the hole. If you do that with your red plug Stemco cap it will be slinging oil. That is where the fill to the bottom of the hole comes from.

 

Check that your axles are level before raising or lowering the oil level.

 

it ain't rocket surgery. if there's oil in there you are good to go. a 1/16th or 1/8th of an inch above or below the full line ain't gonna matter. Oil=good. Definitely new caps and oil and check bearing tension too.