Hello. My S70 suddenly is low on coolant. this has never happened before and I am the original owner since '98. Got about 125K miles on it. i noticed this a few weeks ago and added some of my 50/50 Volvo antifreeze/distilled water mix. Now it's low again and I am out of the mix and my local Volvo place has turned into a local Subaru place (darn!) and I do not have a place to grab a gallon of Volvo stuff. My expansion tank is almost empty, the coolant light comes on when I accelerate or turn a certain way, so I am low. The temperature is fine, maybe one bar higher than normal, maybe not (it seems to be in the middle and not close to red zone). It is very cold out today, but I tried to look under the hood. I could find no leaks, no smoke from the tailpipe, no strange smell inside or wet carpets or anything like that. Car runs fine and I am still getting 25 mpg as usual.

I would like to top off the tank while I troubleshoot, but I don't have easy access to Volvo stuff. Can I top off with something else? I have Volvo stuff in there now.

It's too cold to work on the car for me right now. I hope it's something simple.

Any help on alternative coolant product and trouble shooting recommendations will be greatly appreciated.
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Vladimir. '98 S70 base, 5-speed manual - his, '93 945 - hers