Oil pan leak

[Okie-Go]

I came home yesterday in the wife's truck and noticed oil on the driveway under my truck. The back of the oil pan is leaking. Does anyone know the correct size to tighten them. I tried 3/8 and 12 mm but dont work. About 1500 miles ago I was in a bind and had the oil changed out of town. I am thinking they did not let all of the oil drain and since I gave them the three gallon it needs it is a little too full. I think it is odd after more than 25K miles it starts to leak. Any thoughts are appreciated!