Ain't that the site glass on your air conditioning accumulator?
In the good old days, you could form an opinion on your AC system's charge condition by observing the bubble flow in the refrigerant through that sight glass.
No bubbles could've been an overcharge or a good charge.
A couple bubbles were okay.
A constant stream of bubbles indicated a low charge.
The hiss and the oil you noted indicate a long hot summer soon unless you're going to spend some money.
But Seriously, it could just be any thing from an loose fitting / O-ring to a fragged compressor, so good luck.
Okay okay, maybe it's not an accumulator, maybe it's a receiver dryer. Small difference in this case, The part you are looking at is a reservoir for the refrigerant in your AC system, It should not hiss at you nor bleed stuff.