The "major reason" for changing to a stock 88 Fiero oil pressure sending unit when doing a 3800 swap into a Fiero is so the stock Fiero oil pressure gauge on the dash will work properly and give reasonably accurate pressure readings.
Most stock oil pressure sending units that come on 3800 engines are incompatible with the stock Fiero oil pressure gauge on the dash.
MECHANICAL oil pressure gauges are dangerous. On more than one occasion I have witnessed the mechanical oil line leak or burst inside the passenger compartment while the engine was running, and you can imagine what a mess hot engine oil can make inside the car (not to mention the risk of spraying the driver or passenger with hot engine oil, burning them). A good electrical gauge is the only way to go.
A properly working OE Fiero oil pressure gauge is reasonably accurate. Modern aftermarket electronic oil pressure gauges are more accurate and highly reliable - but they cost more. I wouldn't recommend anyone run a mechanical oil pressure gauge except on a farm tractor.
OVERKILL IS UNDERRATED
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