I thought the recommended oil for all Fieros was 5 W 30. Of course, with any engine, as it ages and wears, a heavier grade might be necessary. On Pennocks, there are reams and reams of opinions, pro and con, for any imaginable steps to take and oils to use. There's probably no one answer that fits all.
Roger's recommendation of a conventional multigrade oil is probably best. Synthetic blends probably being next. That is my opinion because of the type engine we have. They are flat tappet pushrod engines. Old flat tappet engines used oil with lots of zinc which helped reduce wear on the bottom of the lifters and on the cam lobes. You will also find lots of pro and con arguments as to whether zinc (ZDDP) is of any benefit. You can purchase a ZDDP oil additive, but you must be careful to add the right percentage and no be tempted to over-do it. More isn't better in this case. Some of the so-called high mileage oils have ZDDP as an ingredient. They're usually of a higher viscosity like 10 W 40 or 20 W 40, but in my experience they didn't increase my oil pressure enough to warrant the extra cost of regular 10 or 20 W 40. Modern day engines don't need the additives because almost all engines now use roller lifters or overhead cams with roller rockers.
Any multigrade with a rating of SF would meet the requirements. With a higher mileage engine, maybe 130K or more, it might be good to go to a 20 W 40. By that mileage, you should normally already be running a minimum of 10 W 30. Best to put a mechanical gauge on the oil pressure sender tube and get a pressure reading when the car is at operating temperature and idling. I like for that bottom number to not be below 12 psi at 900-1000 rpm.
As for the brand, and again this is personal preference. I've gotten over 300K on a new Grand Am, 225K on a new Dakota, 297K on an original 85GT, and 183K on a Grand Prix using Castrol GTX oil and changing oil and filter every 5K miles. All the vehicles were sold and continued to be driven by the new owners.
Synthetics have come a long way to overcome some of the tendencies to find every little pinhole to escape the engine. Again, there are lots of arguments for either side of that. Like Roger, some say never add it to an old engine. Others say never add it to a new engine until it's been driven for more than 25k miles to be sure it has fully worn every part of the engine. Synthetics do lubricate better and the thought of some is that because it's so slick, pistons, rings, valve stems, bearings and such don't wear and mate if synthetic is used too early.
A good oil filter is a must. Lots have said that the popular Fram filters have begun to fall short in quality compared to Wix or Purolator or Purolator Gold. Probably some searching might help with making a choice of brands.
Do a google search for ZDDP oil additive to form your own opinion on that.
