What should be the minimum antifreeze level in a solar system

Good day. I have the following question about a solar system. Age 5 years Filled with thermosolar The antifreeze strangely measured at -40 degrees! The liquid was somewhat gray-blue. But without flakes etc. Clear liquid, but not transparent. When should one change this?

In my opinion, there is no clear answer to this:

Antifreeze agents are mostly based on different alcohols in an undeclared mix.
Alcohols are also solvents and may possibly attack sealing materials.
However, this does not have to be the case, because usually the material of the seals is also not declared.
Since the fluid circulates in the closed circuit without new air supply,
after a short time there are hardly any new aggressive chemical environments.

The mix can therefore function stably for years as long as no seal is attacked.

Ok, then the solar fluid should still be okay. I find it strange that my measuring device showed a proud -40 degrees, although it actually says -32 degrees on the canister... Calibrated the device beforehand with distilled water. I will now take a sample and measure it every year.