I'm not aware that the '88 vintage 103 has a knock sensor, so it cannot retard ignition advance if detonation occurs due to insufficient octane.
The the EZL ignition system does electronically determine advance, but only uses engine revs and manifold vacuum as input variables.
I've used 87 PON gasoline in my 2.6 and only noticed minor low speed transient detonation, which is easy to drive around by modifying my throttle habits. With only a 9.2:1 compression ratio, premium fuel seems a bit of overkill for these engines, and the use of lower than 91 PON fuel, particularly in cool weather will probably not create any problems.
The use of knock sensors in modern cars is almost universal, and high compression engines where premium fuel is recommended by the manufacturer have two basic electronic ignition advance maps - one for premium fuel and one for regular grade. The default map is for premium fuel, but if consistent detonation is detected by the knock sensor(s) the system will switch to the less aggressive timing map and continue to adjust timing as required to keep the engine out of significant detonation. In most modern cars you will NEVER hear detonation because the systems react very quickly to light detonation that is usually not audible to occupants, but modern electronically controlled engines will often operate at the ragged edge of detonation, which will yield the best thermal efficiency - the best fuel economy. Modern electronic engine controls react very quickly - changing timing up to about 50 times per second, which is every firing stroke at 6000 revs.
I know several drivers of recent vintage cars who switched to regular from premium on my recommendation, sometimes despite dire predictions of engine damage from their dealer service advisors. The less advance, particlarly at low revs will cause a loss of low end torque, which might be noticeable by the most aggressive drivers, but the average driver is not likely to notice any change in performance.
No one I know who made the switch heard any detonation nor noticed any loss of performance, and they continue to use regular fuel in lieu of premium, some for about five years now.