by 
The Curious Bag
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<h4 class="dechead">On One Hand: If Your Cadillac Was Made Before 2000
Before the year 2000, Cadillac Northstar engines were built to run on premium gasoline--91 octane or higher. That being the case, the engines did also come equipped with knock sensors to help cars continue to run smoothly in the event of accidental fill-ups with lower-octane fuel.
On the Other: If Your Cadillac Was Made in 2000 or Later
Northstar engines went through big changes beginning in the 2000 model year. At this time, a coil-on-plug ignition system was added--making it fine to fill up with regular gasoline. Though Northstar engines made since that time run best on premium fuel, regular will work just fine.
Bottom Line
If you have any doubt about whether your Cadillac Northstar engine requires premium fuel, be sure to check your owner's manual. In the absence of a manual, check your Cadillac's title to find out if it was released before the year 2000.
Source:
Autotropolis.com: Cadillac Northstar Engines
CaddyInfo.com: What year did the Northstar convert to regular gas?
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