Creamy oil cap.08 Prius

Could you post a picture? It doesn't sound too good. Do you smell any hint of coolant in there?

 

if the oil in the engine is New oil, then you're probably in for a surprise.

If the oil is not new oil and it's not milky, then you still have hope.

Check the radiator fluid level, not the reservoir level. The radiator cap is under the front engine cover when you open your hood.

 

Don’t ever bother again checking the rad overflow reservoir the rad could be empty and there will be coolant in it.

Take the black plastic cover off the top of the rad it’s 6 screws I think. Push the screws from the back while unscrewing them come right out.
Under that cover is the rad fill cap. Always check the rad coolant level there.
Coolant should be right too the top. Not necessary to put the screws back on that plastic cover it’s not going anywhere when the hood is closed.
I leave my screws out so I can check the coolant level quickly. Just put the cover back on.
Been out for 10 years.

That oil on cap Not normal at all.
Milkshake on the cap may mean blown head gasket. Blown head gasket on
A G2 is sometimes caused by the water pump belt falling off and engine severely over heats or the water pump is blown. Blown pump is easy to spot get flashlight check front of engine real close for red or green dried flecks
That’s coolant leaking out of the pump weephole.

 

Right, probably a lot of sludge in that crankcase if the car never really gets warmed up. That could be what you are seeing on the oil cap. It's probably best to change the oil as soon as possible.

 

That's good no pump leak i assume the belt is on. Its probably the stock belt that must be replaced its very tactical you lose that belt you will toast the motor. I replaced mine at 75K. It looked ok on the pulley but was pretty cracked when i took it off. No temp gauge on these cars you will never know if the belt failed and that engine is overheating until it throws a check engine light. Too late your way past damage temp on the engine. See alot of those over the years. Really stupid design flaw so go out of your way to fix that.

The coolant looks ok to me except that yellow stuff deep in the cap never saw that before so not sure what that is to be honest. If it was me I would take the valve cover off and see what it looks like in there. You can see how well the engine has been taken care of over the years.

Its due for spark plugs so you have to go in there anyway and this engine like most 4 bangers gets a leak in the spark plug holes from leaky gasket. Very very common. Mine was leaking in #4 at 125K.
Before you pull the plug get a mirror and flashlight and look in the spark plug well and see if there's any oil in there. I use a borescope.

The engine takes exactly 3.6 quarts of oil with new filter btw. If you take it to Jiffy or one of those quick lube joints they show the G2 takes 5 quarts in there database so be careful if you use those places. They will overfill the shyte out of it.

I use them sometimes but I bring 3.6 quarts in a jug so they cant mess that up.

Here's a pics of my valve train if you do replace the gasket please take a photos and post them here.
I use very high quality Redline Motor oil 5-30 which at operating temp is a 40 weight oil to fix my minor oil eating like lots of other G2's. Does not affect my mileage at all. As you can see in the pics it does a great job.

 

fomenter, for the coolant density gauge to read accurately you need to fully fill the testing chamber. I as well agree that it is a build up of condensation in your oil fill cap from short trips.

Ed's technical information above is spot on as those that are common around here can attest to.

 

If I were you I'd check your oil, then take it on a long, suburban city trip (maybe even 150 miles in a single drive) with your defrost on so your engine stays on most of the time, then check the oil again. If it looks a little sludgy or different than before you set out I might change the oil again and get the rest of the condensation out. I would even put in a whole bottle of Techron fuel system cleaner in while you're at it, make the trip doubly useful.