Can I put regular fuel in my 1990 caddy eldorado?

most cars that say premium will run just fine on mid-grade gas. You can safely use mid-grade as long as there is no 'pinging' during acceleration. Some engines will also run well on regular gas. Just experiment to find which is best for your engine. The only trade-off is that you may notice a little less power - but that depends on your style of driving. If available, stick with Tier 1 gasoline. They have more additives than the minimum required by federal law so therefore your engine will stay cleaner and run better.

I tried regular and switched back after a few months because the deville started to run not as well, took a few tanks of high test to correct, I will never use regular again

If you went straight from Premium to Regular - I would expect those results. My original post suggested using Mid-Grade which is 89 Octane. Switch to Mid-Grade and stay there awhile. At 89 Octane, it is normally a good choice between the pump 93 Octane and the recommended 91 Octane of some engines.

I have a 1984 Cadillac Biarritz Eldorado how can idle down what kind of gas would it take

My 1985 Cadillac Eldorado was running great on regular gas . yesterday I put Mid grade gas in her and she coughed bad .. This Is nuts She just turned over 70,000 miles. She sits in a garage. She is spotless. She has always been a great runner no problems at all. What could be in this gas.