can coolant and oil "unmix"?

[maka_RTH]

so, i was gonna replace my friends headgasket cuz it overheats, and when i checked it out before i coulda sworn that the collant looked like chocolate milk. except when i finally get around to working on it today(it's been sitting for at least 3-4 days), i drained the oil and the coolant, and both looked just fine. it did seem a little low on coolant(not sure how low). and the thermostat's little air-bleed thingy(on te t-stat itself) was all clogged, but i don't know that that would really do anything... anyways, it does leak oil from the oil pan, and it looks like it leaks from the head(as well as the valve cover seal, but i'm gonna replace that anyways). should i try a new t-stat? then fill the coolant and replace oil filter and new oil, and try it that way? if i remember right she said the temp needle always runs normal, like it doesn't get hot. yet after running it for a bit, it acts as if it's overheating and just dies... sorry it's long, and any help would be appreciated.

[SteveDX89]

No, they can't un-mix. The coolant that leaks into the oil passages will flow down into the oil pan with the oil and sit there. Use a new thermostat since they're only a few bucks and make sure you clean out the air bleed thing the best you can.

[NXRacer]

my coolant in my old coupe used to look like that. It ended up being super rusy (who knows why). No matter how many gallons upon gallons of water i'd flush through the system, there would always be rusy water. A good sign of oil getting into the water is that it starts to look like foamy mayo.

[night]

water + oil = mayo

and black shit in the coolant could be rotting hoses. but generally more bits than just color

[maka_RTH]

yah, i'm startin to think rust, that and now that i think about it, the coolant was VERY low, cuz i got just over a quart out, and it holds over 5. so, i'm gonna fill the coolant and oil change tomorrow, then see how it goes. and if she wants me to, i'll still do the oil pan seal so it won't leak as much oil. but yah. i'm also gonna check plugs and whatnot for basic tune-up.