Engine knock from only slight oil loss?

That's very late in the day. How's the engine coolant level in the reservoir? I'd do head gasket test, boroscope inspection of the cylinders (preferably with cooling system pressurized, looking for coolant tears getting past the head gasket) and/or leak down test (air pressurizing individual cylinders with a special gauge set, seeing how well they hold pressure, and if they leak, where).

New short block is about $2K. If the cylinder walls are in good shape then just the revised pistons/rings would be cheaper. Part no's for the pistons/rings in attached.

 

Pull the plugs smell them look at them with a small loop or good magnification glasses and look for vaporized drops of water or vapor drops on your plugs that will be coming from your engine The only water or that kind of liquid in your engine is in the radiator and the cooling system Stick your tongue to the spark plug you'll taste the glycol if you know what that tastes like It generally you'll be on one and two and not three and four If you have these conditions then your engine is heading to toast just plain and simple that's it If you can verify that you have this knocking going on and it is not because water is putting the fire out like on my 2013 then when this is happening and you're standing outside the car all the noise will be coming right where the engine and transmission mate up. It will be very distinctive You will see your engine running you put your hand on and it doesn't miss a beat but then you hear this clank clank clank clank which is not the springs on the damper plate being jammed to their end of travel because of the fire getting put out and the engine missing. It is something similar but it's not that and now you're transmission has to come out for inspecting that torque plate and it's flywheel looking mating partner and if that all checks out then the output shaft of the transmission needs to be checked and possibly open that up to see if any bushings or bearings or what have you have failed on any of the shafts of the motors or however all that works in there mine is not clunking due to the head gasket that was just fixed and it's been fine for months now I have this clanking and it is not due to ignition or misfiring so I have to pull the transmission to have a look If yours is similar to this and you've done all the other stuff above and you don't have any glycol in your oil and on your plugs then you're probably looking at similar Good luck.

 

So I'm guessing you've changed the spark plugs at some point If you have not now's a good time to loosen up the windshield tray and get the plugs out keep them in order of where they came from look at one and two especially closely see the little water droplets they're real small take your finger and touch those water droppers and touch it to your tongue or smell it It smells like glycol smell your antifreeze bottle to get an idea of that smell but smell the oil or spark plug first or touch it to your finger then to your tongue then smell the coolant however you do it If you have traces of glycol on one and two they're in lies your problem it's starting to seep through the head gasket and get into your bores or your cylinders.

 

The atkinson engine is known to have this problem occassionaly and it doesn't mean anything is wrong... Soon as you hear a sucking sound when you open the coolant reservoir you'll have the same problem that all gen3 Prius owners have to deal with. But until that time, you got nothing to worry about and the car is very reliable, though those high MPG piston rings wear out early which is what you're in the first stage of.

 

The sucking sound means you're losing coolant... Perfectly normal for a Gen3 Prius' bad design... You can sometimes go years with this tiny head gasket leak. Fortunately for your that you live close to Oklahoma where Drew Griffin will put a gen4 engine in your Prius in your driveway in one day for $1500: https://www.instagram.com/skimmilkhybrid/

 

This is the definitive early test if the borescope is multiview and used properly. Catching a hg early might save the engine instead of allowing it to simply run longterm when it’s really bad and obvious to anyone.

Use of a borescope
Car Care Nut HG Borescope at 7:40

 

Like I said... You have lots of time... But the option of buying a used gen4 engine for $1800 and only having to pay $1500 for an installation eliminates future headgasket and oil burning piston ring issues forever. Way more cost effective if you want to be driving the same Prius 200K miles from now. And if that's not your plan, sooner you can sell it the better!

 

Maybe have an independent mechanic (one that works cheaper) do this.

the video in post #11 may include some comment on cylinder wall condition. I believe you’re looking for “scuffing” on the cylinder wall cross-hatch.

Ask some. You likely should be sitting down.

Snake oil.