Are you noticing any oil loss? If so, you have another leak - probably the head gasket.

If you aren't losing any oil, chances are there was some residual oil in there that never got flushed out. Getting foreign crud out of a cooling system is a royal PITA.

I have an old '78 Volare that the PO decided to put stop leak in. Well, I replaced the radiator, the hoses, etc., and flushed the whole thing for no less than 30 minutes with a garden hose and the motor running.

4 years later, two coolant changes later, I still see that crud when I take my cap off. And that's on a car with a dirt simple cooling system.

-tp