You can also reset the ecu by disconnecting th4e battery and then pressing the brake in for a minute or so, the action of doing this tries to light up the rear brake lights and removes all of the stray voltages from the car.
then connect back up and go for a drive.
Make sure the front wheels are straight when you do this or else the hicass can get a bit screwed up because it thinks wheels turning to the left slightly(for instance) it then straight ahead when you've reset it.
this should be done from time to time on all skylines even when it is running ok.
The ECU would just have the mapping for the fuelling etc.. this wont change but the timing and airflow is monitored all of the time and adjusted according to conditions.
if they have been using crap fuel or even SUL that has gone off in the tank then you'll benifit from the ECU reset as it relearns the optimum settings and changes the timing in very large increments initially then as it gets closer to being right it narrows the adjustments down and this is what happens when you change your fuel or put in octane booster.
The ignition timing is advanced until knock is detected and then pulled back. in narrowing swings above and below ideal. until it has narrowed it down to being ok.
once settled it almost stops learning and will use tiny adjustments each time you go out and take a lot longer to get to its ideal each time.
so if you change fuels it is a good idea to reset.
so have the code for your stereo handy