I'm not sure how much of the wrong coolant it takes before it gels up but you may want to consider flushing the system to get that G12 + green out of there. You can then fill it with whatever you'd like. You can also top off G12 with distilled water.

The coolant below the tank that's green was probably from spilling but one thing to watch out for: The coolant tank can develop cracks and will usually leak after the coolant gets hot. (heat and pressure expands the cracks enough to let vapor and/or fluid to escape) In that scenario you'd loose a small amount. Not enough to notice after a day, maybe a week.

I'm in buffalo and it's been wicked cold over here too. I think my coolant flange started leaking from the weather too. It's too cold for me to work outside so I had my mechanic do it a few days ago. It was about $150 total.

 

ok first thing you need to do is get the stuff you have in the system out. then, fill it up with straight water and put some fluorescent dye in with it. after you put the dye in, run the car for like 10 minutes and then look for the leak. from your picture, it looks like you have a bad reservoir tank if it's only leaking from that spot. are you due for an oil change anytime soon? if so, take a good look at the oil being dumped out of the engine, if it's got a funny color and not straight black or slightly brown, your headgasket is out which means you need to stop driving the car right now. if it's the headgasket, you'll also seen funny colored gas coming out of your exhaust. if the gas isn't white, grey or black, it's not good. if it's black it's something that doesn't have to do with the coolant. which chicago are you in?

 

I am in Chicago chinatown. I always drive between a town and Chicago which is about 140miles. The level drops about 1cm after driving 140 miles on highway continously. Is that so bad?

 

well you shouldn't be losing any coolant. i lose coolant when i drive a long way on the highway too. my radiator has a small leak. do wut i said in my last post and get the current liquids out of there and put in water

 

I had a similiar problem recently. It turned out I had a crack on the bottom of the coolant flange. I replace it and it fixed my leak. A few days later the clip that holds the temp sensor failed and it shot out the sensor. If you need to replace the flange, make sure that you get some good metal clips, instead of the cheap plastic ones that audi has for sale.