94 Isuzu pu 2wd you have a oil leak on left side of engine out of aplastic tube that is almost between motor mount what is this?

The tube is from the vent on the fuel pump. Oil that lubricates the fuel pump plunger shaft is supposed to return to the engine by a drain hole in the head casting. It is designed so that if oil does not return but gets into the top of the fuel pump, it is expelled out the tube you are referring to to prevent hydraulic lock of the fuel pump.