Not thet is not an oil, just dirt from the system. My case, I changed antifreez and put on a new antifreeze container, I drove for a while, I mean a month or two and then the dirt appeared. I change Antifreez again a new antifreez tank and now all is well.
So its a some kind of dirt , im talking about my car ,it is a possible that old owner long time ago pour some sealant in radiator , maybe , just maybe , but i did change a radiator , so maybe some dirt left and end up in my container , or its just normal sistem dirt , a normal bicause its an old car .
It may be best to wash the system with some liquid for that, I don't know what's liquid name, but as my mechanic told me to say with that liquid you drive for a week and then that wash the system for you and then you change the antifreez again and put a new container.
Never put a sealant in the car , better fix it . At least the aftermarket part for him is cheap , even a premium Nissens is 110 -160 euro for engine radiator , 80 - 140 euro for Nissens for air condition radiator , this is for example .