Hello and welcome to the forum,
I would use Seafoam in both the oil and fuel system. I have used it for several Fieros over the years, and my other cars also. do a google search for "pennock's Fiero Seafoam" and also search Youtube for how to videos.
When using it in the oil, I use about 1/4 to 1/3 of a can for a v6, don't over do it (less is more, it thins the oil)! When car is warm add the seafoam and let it idle for 5 mins.Then change oil and filter. DO NOT drive the car or rev the motor with the seafoam added, It may take several oil changes to get the motor cleaned out. This is just brief details, you will find out a lot more info in your research.
Remember, this is just what I and others have done. Condition of motors and fuel systems vary and anything you attempt is at your own risk.
Good luck with your projects and please share your results and projects on the forum, it is a helpful and wonderful place!