Volvo 940 - Oil in the Coolant ??

I have a 1992 Volvo 940 with 260K miles. I am seeing evidence of oil in the coolant - it looks like light brown gooey slime floating on top of the coolant in the reservoir - but I am not seeing any evidence of coolant in the oil. Other that that, the car runs normally, no problems. Any ideas ? Is the head gasket on its way out ?