Prior to lockdown Soopra (93 JSpec N/A) was running fine, in daily use.
Went in to lockdown with 3/4 tank fuel (normal unleaded, been using this for 9 years, no problems).
I was starting him every weekend and leaving him running for about 30 minutes
About 6 weeks in to lockdown he developed a misfire at idle when he was up to temperature.
I replaced the ignition coil and this made no difference.
Once I got down to about 1/4 tank fuel I added some octane booster and this fixed the misfire.
Refuelled the car with Normal unleaded. Misfire back
Refuelled the car with Super and the misfire has gone again...
Any idea what the problem is please?