Thank you very much Genome for your Round Wire Calculator. It gives amazing results! (BTW what is the meaning of the items Dia(L), Dia(S), Dia(H) and Dia(T) ?)
I finally found a test to demonstrate such kind of figures. I made an impulse test. I arranged the inductor in a LC parallel circuit (c=10nf) and I used as impulse a DC current suddenly decreasing from 50mA to 0. With the scope I catched the damped oscillation and I calculated the time constant of the exponential decay. From tc=2L/R, using the nominal value of L=12.6uH (measured @f=1KHz) I deduced R and also Q=omega(L/R). I tested also the circuit with different capacitor (C=0.47uf) so to investigate the behaviour at a lower frequency. Here are the results:
a) L=12.6 uH R=0.046 ohm Copper plait made of 8 wires diam. 0.315 mm wound on air.
C=10nf Oscillation frequency = 495KHz R= 6.8 ohm Q= 6
C=0.47uf Oscillation frequency = 69KHz R= 0.32 ohm Q= 17
b1) L=12.6 uH R=0.055 ohm Single copper wire diam. 0.64 mm wound on a toroidal core (Micrometals T68-2).
C=10nf Oscillation frequency = 470KHz R= 1.9 ohm Q= 21
C=0.47uf Oscillation frequency = 69KHz R= 0.19 ohm Q= 29
As you can see those figures are not very different from the ones elaborated by Genome's program. This definitely answers to my original question. My test oscillator requests an higher power supply current with inductor a) because it has a very poor quality factor (that is a more resistive losses). And the main reason of that is the multi-layer structure of the inductor.
A final observation. The inductor will be used together with a C=0.47uf to implement a low pass filter for a class D amplifier. The lower Q of this inductor will reduce the peak of the audio frequency response and it will produce a more flat bandwidth. At lower frequencies the equivalent R of the inductor approaches the DC value (0.046 ohm) ensuring in this way a fair damping factor. I think this kind of inductor could have unexpected benefits...
Thank you very much to all the guys of this thread for their precious hints.
Marco