From your picture, it looks like the oil pump O ring. See how clean it is around where the oil pump cover mates to the timing cover in the corner. and it is dirty to the right of the corner.
But the 3 places it will leak is the oil pump cover, crank seal and the crank sensor.
Yes the crank sensor seal can not be replaced with out replacing the sensor(Toyota does not sell the seal seperatly, a $120 O ring).
While I have tried several things to fix this oil leak at the sensor. As long as the noise of the truck is level or up hill, you will not loose any oil out the hole if you try to repair it. I have taken the sensor out and clean it real well with Brake parts cleaner. Then used the black FIPG on both side of the seal, do not be stingy with it. Do not get any on the sensor face. Clean hole real well with brake parts cleaner. Bolt part in allow a couple of hours for the FIPG to set up(it will skim in 15 min), you want it to cure real well before you put crank case pressure or oil on it.
This has worked well on several applications where the sensor still is working and is cheap, compared to the price of the sensor.
Other products that I would think would work as well, The Right Stuff, Copper RTV by permetex.
Good luck.