What does oil in the radiator mean?

I just got a Honda cbr 600 f2 and it had a crack in the radiator cap so I look in it and it had oil in it. I just bought the bike and it runs and rides fine but It has oil in the radiator. I called the guy and asked him and he said he thought it was cracked but he said a mechanic told him it was a seal that was bad. I drove it home and the dude drove it to meet me which was about an hour drive for him. And it rode fine. I'm just trying to figure out what I need to do to fix it. Thank you

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