Oil weeping around oil pressure sender.

Clean the area and wait until your next oil change. If you can pinpoint it as the culprit, I would drain the oil, remove the sending unit, use some Loctite, screw it back on, then replace the oil filter and fill it up with oil.

 

The Repair Manual specs a specific adhesive applied to 2 or 3 threads:

Toyota Genuine Adhesive 1344, Three Bond 1344 or equivalent

Then, to install the unit within 3 minutes of application, and don't run the engine for one hour.

Also says to install with a 24 mm deep socket, with torque of 11 ft/lb.

Some discussion of equivalents here:

THREE BOND 1324 or equivalent to Loctite Blue 242 - YotaTech Forums

Or just ask at Toyota Parts Department, maybe they can get it?

 

Here's a pdf of Toyota adhesive equivalents, cobbled together from a link in the above.

 

I found the same thing a few months back - I was looking at the AC I think & noticed a dirty/black area at the firewall side of the engine on the passenger side. After cleaning the area with paper towels I could find no "leak", so I let it go.
A month or so later I looked & the area was oily/black again. This time after cleaning the area I tried to pinpoint the leak. It is impossible to see clearly in that area but I guessed it was coming from the back of the sending unit. Looking on Google it seems the Toyota oil pressure switch is known to leak as it gets older. iirc this was across all the Toyota models.
I ordered a generic replacement & have it on hand for when I make time to install it.
I just checked the area on my Prius. Again there is a small area around the oil pressure switch that is oily/dirty. It appears to be a very tiny seepage as there if never any oil on the engine undercover.

 

A617 - I just looked at your pic. My sender was about that dirty the first time I noticed it at ~120K. After initially cleaning the area it barely gets dirty after 1-2 months. Just now I cleaned the area as best I could & pulled the connector off.
The connector is an oval shape with one rounded end & the opposite end has a flat across it. This flat area has a tab that must be depressed to release the connector. The single pin electrical connection has an "O" ring to shield it from oil/dirt. Nothing looked out of the ordinary & I just snapped the connector back on.
Mendel - thanks for the adhesive research!

 

Leaking oil senders were very common in the 1960's and '70's.

You needed a special socket to install them without bending the outside hex housing, If the thing housing is bent, it will again readily leak.

You don't need a special sealant. Pliable Permatex brand "Form-a-Gasket" or any pipe dope will do on the threads.

I don't konow about the current Prius models, though, since I have experienced it in mine.

 

Right. The oil pressure switch of my '81 Mazda abruptly sprung a bad leak, which I luckily noticed in time to avert disaster. I used the least expensive sort of pliable gasket sealant on the threads of the replacement, and never had more leaks on that front in the following several hundred thousand miles.

No leakage from the one on my Prius, so far.

 

I know this is an older post, but since these live forever on the web someone may reference this post in the future.

I got home from my long commute and noticed oil on the ground in my driveway. I initially thought someone parked their leaky old car on my driveway while I was at work. There was not a trail of oil leading out to the street and so I dismissed the possibility of it being my car. Then I thought I’d better check underneath the car just to be sure. There was oil everywhere.

I immediately drove the car onto my ramps and checked the oil drain plug and the filter. Both were secure with no signs of leakage. Oil was present on almost every other surface however, including the suspension on the passenger side, all over the rim and tire, and all over the undercarriage. I could not find the leak initially. I made a call to my dad who was a mechanic in a past life. He had me check things and one by one we crossed them off. Then we got to the oil sending unit. It wasn’t immediately obvious that it was leaking, but when I turned the car on, the drips immediately started.

Luckily for me we caught it quick. I only lost about a quart of oil, but what a mess. I highly recommend just changing it out if you’re over 100k and haven’t had any issues. My car was at 145k roughly when this happened. The part is available almost everywhere. I got one at O’Reilly with a lifetime warranty for $14 and change. It was pretty simple to install, and only required a bit of creative contortionism. I see on the web there are easier ways to access it (by removing the passenger wheel I guess,) but I was able to do it from below.