It depends on what the octane ratings of the grades you use, which vary from state to state in the US. A rating of 87 is "regular" in most places, and the owner's manual for the 124 Spider cautions against using fuel with ratings below 87, declaring that this could damage the engine. There may be places where 85 octane is "regular." I hadn't heard before that switching between grades was bad for the engine, unless you switched to a lower (87) octane where pre-ignition knock might intrude. In that case, the engine would be "adjusting" more during one tankful than in making a switch to a higher grade. But I'm no expert.
Premium vs regular
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In the US and Canada the "posted" octane (what's posted on the pumps) is "road" octane, the average of RON (research octane number) and MON (motor octane number) whereas pretty much everywhere else in the world the key octane number is typically RON. The two octane number tests are similar and differ only in the speed (rpm) at which the test is done. In Europe "regular" is 95 RON and although there is also a spec on MON, for most gasoline formulations at different refineries, RON is the limiting constraint. 95 RON is what the Fiat and Abarth Spiders are designed to run, with no benefit from running higher RON unless the ECU mapping is set up to adjust advance and other parameters to push up against a knock constraint. I run my modified Fiat Coupe on Tesco (supermarket) 99 RON because it's been mapped specifically for it, whereas the standard Fiat Coupe I just bought is run on 95 RON. I will run the Spider on 95 RON and will buy the cheapest available supermarket fuel. Anyone who works in or lives near a refinery knows that all the different "brands" come from the same limited number of refineries, with differences only in additives perhaps. Running higher octane in an engine that can't take advantage of it is a waste of money IMHO, it's just a placebo thing. And BTW I don't work in a refinery but work as a consultant planning new refinery and petrochemical facilities so I like to think I know a little about the subject but as you guys in the US say, YMMV 
Here, in Montreal, we use to have several refineries, then to 3, now only 1. and they supply all the brands(Esso, Shell, Petro Canada and ect.) Before each brand use to have its delivery trucks, now same trucks deliver to all brands. figure that outIn the US and Canada the "posted" octane (what's posted on the pumps) is "road" octane, the average of RON (research octane number) and MON (motor octane number) whereas pretty much everywhere else in the world the key octane number is typically RON. The two octane number tests are similar and differ only in the speed (rpm) at which the test is done. In Europe "regular" is 95 RON and although there is also a spec on MON, for most gasoline formulations at different refineries, RON is the limiting constraint. 95 RON is what the Fiat and Abarth Spiders are designed to run, with no benefit from running higher RON unless the ECU mapping is set up to adjust advance and other parameters to push up against a knock constraint. I run my modified Fiat Coupe on Tesco (supermarket) 99 RON because it's been mapped specifically for it, whereas the standard Fiat Coupe I just bought is run on 95 RON. I will run the Spider on 95 RON and will buy the cheapest available supermarket fuel. Anyone who works in or lives near a refinery knows that all the different "brands" come from the same limited number of refineries, with differences only in additives perhaps. Running higher octane in an engine that can't take advantage of it is a waste of money IMHO, it's just a placebo thing. And BTW I don't work in a refinery but work as a consultant planning new refinery and petrochemical facilities so I like to think I know a little about the subject but as you guys in the US say, YMMV
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