If you had oil hubs without any oil visible you would most likely have some hot or ruined bearings
For those that do have oil hubs the visible ring around the cap is the fill level as marked or molded in the polycarbonate
You don't want 90 weight oil around or up to the hub oil plug
No hole or pin price is necessary either
If its running that hot you have other issues
Ran oil hubs more than 60 years on many different types equipment , never would dream of poking a hole in a moisture and dirt plug
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