Today the airplane has been seen by a specialist.
A good quick test in his opinion was the following:
At sea level (we were at 600') with 40" MAP a fuel flow of at least 22
Gal/Hours should be metered / assured.
The left engines was going at 22, the right one at 23.
The left seems to be a little bit leaner (even at low rpm) than the
right, but in his opinion the airplane is in airworthyness.
The engines tuning to run exactly the same values may not be a good
idea (he said) since they are not in the same
manteinance state (0 vs 1200 hours). Some difference can be seen.
They can also be tuned to run exactly in the same way but this will
last only for few hours and then the differences will come up again..