2005 WRX - Oil cooler leaking water?
 

I changed the oil yesterday and noticed a green stain around the oil cooler. I'm assuming the cooler contains copper and I'm seeing the green copper corrosion. As far as I know that doesn't happen with oil! The two coolant hoses seems well attached, and the green (corrosion?) is higher up than them anyway. A few ideas:

- It's an oil leak and turning green for some reason -> change the oil cooler seal. Easiest fix, but I doubt it's that.
- The oil cooler itself is leaking water from somewhere, maybe through the walls/tubes to the external surface. No idea... replace cooler. $$$
- There's a water leak higher up, but I couldn't see anything like that all. Any ideas?
- It's water splashed up from the road. I can hope, but I don't think it's this one unfortunately. Happy to be proven wrong!

http://i68.tinypic.com/210it0g.jpg

I've had the car just over half a year. I changed the coolant soon after getting it and it wasn't clean, but didn't look tooo bad. There are a few different leaks around the place, and I should've been suspicous of the overly clean engine bay when I looked at it. Too late now! Any ideas about this one would be very welcome. I've read loads of threads about oil leaks from the oil cooler, but haven't seen any about water leaks. Thanks.

Oh yeh, I was worried about the head gasket leaking coolant, but it looks reasonably clean around there. I tried to catch it in the photo so you could see it, so any thoughts about that would be appreciated too.