Cadillac 1958 with the compression ratio of 10.25:1 should not run on gas below 98 octane.
87 octane
Only high octane gas can be used around 92
87 octane should work, although check your owner's manual
With 99 octane gas: Single carb 5 deg Before TDC / Three 2brl carbs 10 deg before TDCWith 98 octane gas: Single carb 2,5 deg Before TDC / Three 2brl carbs 7,5 deg before TDC
The 3.6 engine can take regular (89 octane)
Best but is to read the user manual which states 89 octane
Premium Unleaded 91 octane fuel.
If you are refering to regular gas, it's not recommended. Some people use it as well as myself due to the fact of gas prices. What I do is add a gas octane boost to compansate for the lower octane fuel.
No. 91 octane is the minimum. I notice a huge difference in the way a Cadillac runs on 87. I always put 93 octane in Cadillac Fuel systems.
The best and only gas to use to keep your caddy emmission clean is 93' octane. preferably from a Shell's gas station.
Cadillac recommends you use Premium fuel of 91 octane.
93 octane works best.