Check the pcv valve. They gum
Check the pcv valve. They gum up and allow crankcase oil into the intake manifold. Remove the large black air line above and connected to the throttle body and manually push open the throttle valve. If you see oil pooled in the intake manifold then your pcv valve leaks. The oil will then accumulate and eventually fouls Your map sensor reading. Then you get mpg drop of 5 to 10 mpg. Your oil is pulled thru the intake and is combusted. No noticeable tailpipe smoke because it is gradually doing it. Catalytic converter could be helping also. So you keep adding oil. A new pcv valve is cheap at $10-20 and a diy. HOWEVER....Carefully!!!! Remove and clean the throttle body map sensor with sensor cleaner spray. It's a $200+ sensor. But you need to remove the intake manifold and clean everything else also..... Manifold, intake valves and ports, hoses, map sensor...etc that are oil and carbon covered. It's a long tedious job. YouTube prius intake and EGR and PCV cleanings for details. And add a oil catch can to the pcv line. Do not use a breather on the can, keep it a closed system to keep proper pressure. They are $30-40 on Amazon. The can will help keep your manifold clean in the future. At 100k to 150k you can EXPECT to need to give these a good cleaning on the prius.... Poor design unfortunately. The catch can will allow you to spot a poorly working pcv valve. And Do Not!!! overfill the prius oil as it can enter the pcv valve easier due to the valve being located so low on the vehicle. Again poor design. Hope this helps. P.s. doing this cleaning especially the EGR tube and valve and EGR cooler assembly also keeps the engine running cooler. Excessive heat will blow head gaskets easier, which is also an issue on priuses.