PSD fluid?

OK, I give up. What is PSD fluid?

 

Transaxle fluid in the Power Split Device. You can remove the level check plug and stick a finger in. Fantastic pictures for a Gen 2 prius at hobbit's site. See his 50K and 100K maintenance pages.
Hobbit's techie-rants
There is no dipstick.

Of course, your Gen 3 (as mine) will differ from the pics, and you can subscribe to the maintenance manuals through toyota's web site. The transaxle COOLANT plugs will be down there, also, so be aware.

 

If you want to check the transaxle ATF fluid, raise up the car on four jackstands so that it is level. Find the transaxle fill plug which will be adjacent to the halfshaft that powers the RF (passenger side) wheel. The drain plug is also on that side of the transaxle case but lower. It is not on the bottom of the case as was true with Classic and 2G transaxles.

Using a 10 mm hex key socket, remove the fill plug and insert a finger. You should feel the ATF fluid at the bottom of the hole, no more than 0.4" (10 mm) below the hole.

Tightening torque of the fill plug is 29 ft.-lb. The Toyota repair docs recommend using a new aluminum washer each time the plug is removed and replaced. When you are ready to drain and replace the ATF fluid, capacity is 3.6 US quarts.