Where is the oil sending unit on a 91 S-10 pickup 4.3 v6?

The oil pressure sending unit on a 1991 S-10 pickup with the 4.3L V6 is located on the driver's side (left side) of the engine block, near the oil filter. It's a relatively small unit, often with a single wire connection. You'll likely need to look closely, possibly using a flashlight, to spot it. It's usually screwed into the block and may have a protective cover or be somewhat hidden behind other components.