How common is Gen 2 Head Gasket Failure after 300,000 miles?

2009 with 318,500. The previous time I checked the dipstick, maybe 200 miles ago, oil still looked new yellow from the most recent oil change.
I made at least 4 dozen or more 1.5 mile trips in cold weather (almost all between below freezing to well below freezing). Not even one longer trip to cook off any condensation (Yes, I know that's not good).
The most recent time I drove home the 1.5 miles (45mph max) the touchscreen didn't power on, but there was no red triangle or service engine light in the cluster.
Next time I started the car I got the red triangle in gauge cluster and an oil can light at top left corner of the touch screen.
Oil is no longer yellow but looks milky and oil level has increased from little over 1/2 way between the marks closer to the full mark, so more than a trivial amount.
Next day I started it couple of times just for a few seconds and no warning lights.
My scanner (Xtool D7S) reads no engine codes at all.
I assume that low oil pressure should cause a code? How long does low oil pressure switch reading need to exist before a code is logged?
There is NO WAY that condensation over maybe 100-150 miles of 1.5 mile trips in well below freezing weather could noticeably add to the engine oil level, right?

(I have owned the car for the last 50k miles. Aside from 3-4k oil and filter changes, 1 CVT fluid change and 2 air filters, injector cleaning spaying brake fluid and 5 spark plugs, I have stupidly neglected rest of the engine, including EGR and the cooling system, other than checking the coolant resistance to freezing.)

 

if a head gasket leak on a Gen 2 is from coolant to oil, will a chemical head gasket leak tester like LISLE 75500 show anything?

 

I don't notice any signs of oil in the coolant, but I really should have gotten around the changing the coolant during my ownership because it has never been the color of fresh clean coolant.
Good idea with the dye. without adding anything to either fluid the coolant looks like "pink milk" or Peptobismo. especially under the radiator cap, but elsewhere also, for example on the water pump and around that area there are dried splahes of the same pink milk color.
The engine oil appears as "white milk" on the dipstick and "light blue" on a towel under the UV light.
I assume that new Toyota coolant does not come with dye (but I have not yet tested how it looks under the UV light I have), but my guess at this point is that at some point dye was added to the coolant in the car.

(I meant PCV instead of "EGR". And it turns out that i was wrong, I did get one from rockauto,)

 

Unfortunately I cannot say with 100% certainty if I'm missing coolant as of late, but I would say that it's at least a possibility.
Because the oil light came on, I don't plan to drive it until I at least get an actual oil pressure gauge on it, and probably also new fresh oil, even though the oil in there has only 325 miles on it. Normally my oil is yellow to the naked eye at 325 miles, not milkish looking.

 

Yep, someone I know has a Gen 3 with head gasket failure. His son was using it and it got to the point of empty coolant and ugly noises. I had heard of the gen 3 head gasket issues and told him to come here and research. I don't know if he did.
He had a mobile mechanic replace the head gasket (and timing chain set, head bolts, water pump) and they were able to go for a test drive but I have no idea how long a test drive and if on the engine or battery only until the car quit in the middle of the road with hybrid battery error. I don't know if the hybrid battery was getting charged.
A hybrid battery guy came and replaced his hybrid battery. The guy said that he personally would have much rather put in a low mileage engine.
with the replacement hybrid battery the car still refuses to start and sits after a head gasket, 12V and hybrid battery replacement. I don't know if he is doing any research. He doesn't wrench on cars.

 

Mine does use oil but I keep on top of it so it has never been below the low mark during my ownership, so the oil light did not come on because the oil pump was sucking air. (and it did not come on the next 2 times I started the car).

 

I need to put an oil pressure gauge on it before driving just to make sure that I don't spin bearings and make the issue even worse. (Although a replacement engine rather than a rebuild would probably be the route I would go, so it might not matter, but I still want to read oil pressure before driving to reduce the changes of getting stranded during the test drive.)

 

Just to clarify, the coolant looks like what I described under UV, not to the naked eye. To the naked eye it just looks like Toyota coolant that could use a coolant change, not milky. Tonight I'll look at new fresh toyota spec coolant and the hybrid system coolant in the car to see how they compare to the engine coolant under the lamp.
No wetness (or crystals) on the water pump, just dried out "splashes/spots". I have not looked under the car.
There is coolant directly under the (metal) radiator cap and in the reservoir, so the coolant level did not "get low" during my ownership (about 50k miles).
when replacing the head gasket the head needs to be checked for warpage of course.
If the oil pressure checks out, I might still opt for head gasket only because I might not keep the car very long.
I have never been inside a Prius engine. Only small block and big block Chevy. Automatic transmissions are the only thing I don't work on. I watched a youtube video and the head gasket replacement didn't look too bad, just will take some time.

 

The fact that the water pump has those pink splashes is actually somewhat encouraging because this concern is considerably less than 325 miles old because that is when I last changed the oil and I looked at the oil sometime after that and it still looked like new oil and the dried spot on the water pump are almost certainly much older than 325 miles.

However, a head gasket failing from coolant passage to oil return passage only should be much less likely given the much lower pressure than combustion pressure.

The noticeable change in oil appearance and an increase in oil level, of course, is not encouraging at all.

 

Depends of course on how soon you change. It just looks like coolant that's no longer brand new.

I have not added any dye to the coolant, but there might be dye in it. Or contamination.

I think my next step is to add the oil pressure gauge that EVERY car should come with form the factory and go from there.
I'm still puzzled why the oil light came on but left no code and no check engine light, unless they put in some code requiring it to read low for x number of seconds before tripping a code to filter out false readings and I shut it off too soon.

 

Just a thought... Because of the water pump arrangement, a leaking RTV seal (at the wrong spot) on the timing cover could provide a coolant to oil pan path... Is this a known failure mode on Gen 2?
Especially if someone had the timing cover off on this car and didn't apply RTV properly, this could be a possibility.

 

Just to clarify, the oil looks like "white milk" on the dipstick under the UV light and light blue on a paper under the UV light.
And the coolant looks light pink under the UV light, both under the radiator cap and on paper.
(And there is no dye added yet. Dye could have already been in the coolant, I don't know.)

Yep, the potential for damage is why I'm not doing anything that involves driving/warming up the engine at this point.

If all I do is change the oil and coolant, I think that I'll be exactly where I am now very soon, as long as the many many 1.5 mile trips in very cold didn't cause so much condensastion to collect that it changes the dipstick level, which I don't think it could.
(Driving only many 1.5 mile (per google maps) trips with full cool-down after every 1.5 miles is something I have never done before).