Are Oil Free Air Compressors REALLY Oil Free?

There is a way to add the oil back in .... i just do not know how. I would have to look.
The oil was put in there when the compressor was built.. or installed so somebody put some in.

The only concern is where it leaked out of. If its cracked then not good.

I am thinking there should be a pipe plug with a square head or an inset allen head. It could also be a regular hex...