Why would a 1987 mustang gt have sensor in the side of oil-pan?

A 1987 Mustang GT wouldn't have a sensor in the side of the oil pan itself. Oil pressure sensors are typically located on the engine block, near the oil filter, or sometimes on the oil pump. There's no standard OEM sensor application on the oil pan for that year.

If there's a sensor in the side of the oil pan on a 1987 Mustang GT, it's almost certainly an aftermarket addition, possibly for:

* Oil temperature: An oil temperature sensor is sometimes mounted in the oil pan for more accurate readings than a sensor in the engine block. This is a common upgrade for performance applications or monitoring.

* Oil level sensor: While not standard in 1987, some aftermarket systems provide oil level sensing. This would require a sensor in the oil pan.

* Incorrect installation: It's possible a sensor for a different system was installed incorrectly.

In short, its presence is highly unusual and suggests a modification or mis-identification rather than an original factory component.