the oil temp. sensor is on the left side of the oil pan, the oil level sensor is on the right side of the oil pan, the oil pressure sending unit is if you have a 4.3L vin "w" is on the top left rear of the engine near the distributor, if you have a 6.5L vin "f" or"s" it is located under the intake manifold on the left side of the engine
First of all a 99 silverado cannot have a 6.5 diesel it it.
The oil pressure sending unit is at the rear of the intake manifold mounted into a fitting going into the engine block. The low oil level sensor is in the oil pan.