Would a half tank of 87 octane gas cause the "Restricted Performance" warning on 1999 Jaguar XK8

If that does not cause the warning to appear what could be the problem

2 answers
if i remember correctly, jaguar usually runs a higher octane fuel. it has something to do atmospheric pressure inside the combustion chamber. it may cause the sensor to over react. shouldn't be a prob. but recommend upgrade air intake to higher air flow system if using a lower octane fuel. even just a high air flow filter.
Should not cause warning. I would have it checked out asap so not to do more damage if there is a simple problem now. Call 313-562-7022 or schedule on the web at StansGarage.net. We will be happy to help you out. Larry