C70: Engine running cool, oil in water

[Alby]

Hi guys. For a couple of weeks I've noticed the car has been running cooler than usual - temp gauge usually sits at half way mark but recently has been a couple of notches below. Yesterday on the drive to work it barely got past the quarter mark. I checked the water level and the expansion tank was almost empty (no warning light which was strange, too). Filled it up and noticed oil spots in the water - never a good sign.

I checked the oil level and that was low, too. I topped this up a few weeks ago so this isn't normal, either. Today I ran the car with the expansion tank cap off and there were no bubbles and no smell of gasses. There's a little bit of gunk of the oil filler cap but not what I'd call abnormal and none in the oil filler area. Diptick looks clean with no obvious signs of water. Engine is running fine and gearbox changing ok so my question is: Head gasket or radiator oil cooler?

I'm inclined to think it's the oil cooler but the engine temp thing has got me questioning myself. Surely if it was head gasket it would overheat rather than run too cool? Could oil in the water affect the thermostat working properly or is it just as likely to be a separate fault?

Any input is gratefully accepted.
Car is a 2004 C70 2.0 turbo auto convertible.