Not sure but I'd trash it! Fluorocarbon all the way, it's pricier but $ well spent. Who ever spooled that must of had greezy azz hands. 😆
It sounds like some kind of oil to me.
The major trouble with mono is UV as said even if it had never been opened direct sunlight can break down any mono but that would be on the outer layers.What you could try just to test the line is to put a bowl full of water in the microwave for 2 mins.Take your spool place it into the hot water for about 40 seconds to a minute then try a knot.If it still happens the line has been degraded somehow.Another method is to use reel magic and soak the line with it allowing time for it to soak in then try a knot.It may need conditioning but if niether work I would say it is bad for sure.
