It could mean that the oil cooler in the rad has a leak, if you're still using it. If so check the oil level in the motor to determine if and how much got in the cooling system.
The bitch is that it takes a lot to clean the system [cooling] if there is oil in it. I had to flush mine numerous times before there was no oil left in the cooling system and with all the flushing it took out my original heater core, so beware if you have to go down this road.
You will also have to either eliminate the oil cooler, or buy a new rad depending on the age/condition of it.