50/50. It's what I have always done, more so for lubrication and corrosion protection. There's a lot of aluminum in a VW. It's 91F at my house right now. The coldest I expect my VW to see is low 20s. It's hot and sunny and I don't like it. I wish it would just rain. I should move to your part of the world.
I think that changing your coolant every 6 months is excessive and that you are making extra work for yourself for no real reason. Use the manufacturer specified ratio that corresponds to your climate. This is usually 50/50. I also use some type of extended life antifreeze for everything. I think it makes a noticeable difference in the amount of corrosion you find in the system.
Are you actually experiencing overheating problems in hot weather? I don't think you are going to see but a small difference due to the ratio. IIRC there is more than one radiator size that fits our cars. Use the largest (Passat???) one. Use an oil cooler. Get a less restrictive exhaust. Dual fans. How about wiring up a manual fan control switch? These make a noticeable difference.
Not that I want to change the coolant every 6 months...it's more so the fact that after i swapped over to the coolant, from a flush. I experienced my heater core leaking.
Since I don't want to do the work right now, i did something simple which was bypassing the entry and exit hoses from the heatercore entry on the firewall.
The car runs totally fine. No overheating, the coolant sensor turns on the fan when it's supposed to. The thermostat is a hotter one, and the waterpump runs just nicely.
Without having a proper external temp gauge, the dash mounted temp gauge is reading the coolant at the midway mark.
In the city, The sensor kicks in the fan at just above that, and sometimes when it's continuous stop and go traffic it gets to as high as the 3/4 bar but never higher. It's annoying because the fan runs constantly. It's doing it's job that's for sure!
When I was running straight distilled during the flush it wouldn't lean that high up, however the ambient temperature then was a lot cooler than it is now.
While driving on the highway, the dash mounted temp gauge would run below the midway mark, exactly how i expected it to be. Keeping the car warmish to ensure that the combustion is effective.
I've never really played around with changing of the coolant ratio, but doing a little bit of testing to find that right mix is fine with me.
It's been enjoyable working on this.....and now that i've tweeked enough to find out what works, and what doesn't it's almost on to another car