i did a compression test on a vectra that was losing water
all cylinders had a similiar level of compression and it was in the high 15's
week later the car wouldnt start cos the throttle body was full to the rim with milkshake, every single component related to the fuel / intake was covered in milkshake
there was no real warning signs as the water was clear and the oil & level was fine
i picked up on it cos the exhaust was blowing out a sweet white cloud and i noticed that when the car was running rough it had a nice green / blue smoke coming out that was cured when i let pressure out of the coolant system
my other vectra is leaking about a 1/4 of the expansion tank every month and u can smell coolant from the exhaust but that has a 196k on the clock with original HG so it is to be expected
doesnt matter what fluid is in each cylinder it will effectively seal the rings to give decent compression
i would go with head and or head gasket
if your rings were playing up and were not sealing properly you notice it breathing heavy from dipstick and cam breathers
taking the inlet manifold off as you probably already know is far easier than taking the zaust manifold off and you could find out if it is the stem seals within under an hour