Did you try the GM seal tabs like Nick mentioned earlier? You can get them from a dealer if not else where. I don't know if the glycol will vaporize at such low temps, so that may be why you are only seeing water and oil in the breathers and else where. I had a leaking head bolt before, and it looked like what you are seeing. I had enough to up the level on the dipstick a little though. I did not use the seal tabs, will not make that mistake again. The oil on the dipstick looked fine when this happened. You may try letting a small amount of oil out of the drain plug, the coolant would settle to the bottom. Hard to tell from the color with everything mixing, but the you tell from the viscosity if it is coolant. Just let a little out. I pulled my motor back out after the leak and took it apart, other than being gunked up, everything looked OK. I live relatively close to Dave Husek, I was having him do my trans, and he took a look at my disassembled engine, and thought it looked really good considering it was a 100k short block. I put it back together only changed the rod bearings which weren't to bad others were in great shape, so far so good. So even if you got a little coolant in the oil, stopping the leak and getting fresh oil in, you would likely be good to go if it turned out to be a small leak.
