Solid wood parquet on underfloor heating

No, both should be equally warm.

yes, that may be. I did not measure. It just feels a bit warmer.

I will glue parquet (not solid) almost everywhere.

The MEP planner has adjusted the planning of the underfloor heating accordingly and specified more/closer spacing. I think that efficiency losses can be counteracted by proper planning or that the heating circuits can be designed so that the supply temperature does not need to be high.

You have to be careful with such statements of course, whether it was an old underfloor heating with higher temperature or already low temperature. ...

I unfortunately cannot answer that. It was definitely a gas heating system. There were no radiators in the entire house.

On the internet, one often reads that solid wood parquet is rather unsuitable for underfloor heating? Is that true?

This statement is wrong as a generalization. There are types of parquet that work well with underfloor heating. Not every installation method is suitable; full adhesion, for example, works very well because there is little air entrapment. In terms of efficiency, not much is lost, the system just becomes a bit slower to react and the heating must be sufficient to warm the floor through, which should not really be a problem since, besides the flow temperature, the pipe spacing can also be considered as a parameter. Of course, there are also unsuitable processing and installation methods.

What about the recommended layer thickness for parquet with underfloor heating? I have read that without underfloor heating, 8 mm would be good so that it can also be sanded...

The thinner the better. It also depends on how often you want to sand it down.