I get it as well... it'* been like that forever tho.. and it'* always just a little bit of brown at the top of my coolant. If the motor has been running hot it'll be worse next time I check, otherwise it'* nothing really. I've done the intake gaskets, and it'* not that, and the trans cooler was pressure checked when I did the motor/tranny and I had that problem on the old motor too. A head gasket will bubble if oil is getting in the coolant, and it will not just be a little bit, it'll be a lot and get worse with time. I found when my intake gaskets blew, that it was more a problem of burning coolant than oil leaking in the rad, and the car never ran funny... just eventually wouldn't keep cool and everytime I floored the car there was a stream of coolant behind me towards the end of me pushing the time.
'89 SSE - GX3 -190k, 155k on body - Back for another ride.