1998 Cadillac Deville `Gasoline type and grade

I believe the car will run exactly the same as it does when you run the more expensive "premium" gas in it. I have a 1999 Deville, and I too was running premium at first, till my buddy who sells Caddys for a living told me not to waste my money on premium. Now if you can in your area, I'd suggest avoid using any gas that contains ANY Ethonol I used that in a pickup once, and had to have the entire throttle body rebuilt.
In your Caddys manual, it recommends premium, but it states as long as it is at least 87 octane, you are ok. It goes on to say it might make the engine ping, but my Northstar has never pinged on regular unleaded.
Hope I've helped.
Jack