Should i have pressure in water tank when just done a run of 20 miles,,when i release it or it it head problem c180 ,,2005 slight cream on oil filler cap to

When the gas-cap is plugged, you build up vacuum in the gas- tank and cause the gas flow to stop.
Use premium gas only, ETHANOL in the mix is attracting moisture/water which is part of the problems.

If you get an overheating problem, the pressure in the system increases, The radiator cap has a pressure release valve to stop further damage to the system and release the pressure which gives the impression of coolant leaking out. Possible causes of overheating:- Low coolant level (a leak in the system). Thermostat not opening (need to replace). Cylinder head gasket leaking (need system pressure tested) indication of head gasket can be water in the oil, look in the oil filler cap, if there is a creamy brown sludge, indicates water present. Weather if the weather is hot, possible electric cooling fan isn't working. There is a possibility that the seal on the radiator cap is at fault (rare)

Not sure if you had an oil pressure issue and is why you used synthetic oil. Was there an oil pressure issue to start with? Is it possible the oil that is in the water tank... transmission fluid? And is caused by an internal leak within the radiator and is leaching into the cooling system?
Your radiator should have transmission lines running to it, and has a large area for engine coolant to run through it, but also has a small area internally that cools the trans fluid?

I had the same problem on my 9-5 vector 3.0 TiD with the top up coolant warning. Nearly became bankrupt trying to diagnose the problem and trips to Saab. Header tank, sensors, pressure tested etc. The Saab squaddies couldn't figure it out until it got worse. It's a head gasket failure despite passing pressure tests initially...sounds like it's still early days on yours. Had to replace the engine for near £5000.but under warranty :-) Reckon 2.5-3k at a good specialists.

Sounds like you have a blown head gasket or a cracked block, pull the plugs and see witch one is wet that will be the cylinder that is leaking.

this sounds like your head gasket has blown this would be the reason why you have cream colored water, this is oil in the water and loosing water is also a sign of head gasket , a creamy residue on the oil filler cap is also a sign, overheating is defiantly a sign if you have the other 2 all three points to head gasket, you will possibly need the head skimmed now as well...hope this helps

Replace the head gasket or get the block checked for cracks

hi, to me it sounds like the oxygen sensor needs replacing if it runs fine when cold then plays up when it reaches temp then this would be the sensor as the sensor only starts working when it reaches running temp

Check the oil for signs of water if its like chocolate ice cream then this indicates head gasket failure

check water for signs of oil this would look lik mayonaise floating on top in the water filler bottle.

Whilst its running check the water level doesnt rise if it does then head gasket gone if it remains the same then you should be ok.

When it gets to temp and cuts off remove the water filler cap and then try starting if it starts then this is head gasket failure

If none of those show up then i would go with the oxygen sensor replacement.

Let me know how you get on or if you need further assistance ok

check the water pump.Then the fan then claen out the radiator.If all fails Your head gasket has blown.

with the cap off when you start the car does the water shoot out? if so the head gasket is blown and when the cap is tight the water is pressurized by the leaking head gasket and will cause to overheat from pressure blowing all the water out of the overflow. or the head gasket leak is boiling the water in the head and causing the water to overheat.
this is my guess or you have a coolant leak somewhere over the exhaust and it isnt making it to the ground from evaporation.
check for swollen hoses. if you have swollen hoses ill bet they are covered in oil this will break down the rubber and weaken the hoses keep all oil leaks from getting on your hoses at all times as this will destroy them.if you need to replace the cap then the pressure release in the cap is worn out or stuck and the pressure isn't reaching the over flow. replace cap and see if problem is resolved.overheating isn't usually caused by a cap.
but cars differ in design and the thermostat may need the pressure to release properly.
if that doesnt work try replacing the thermostat it is responsible for releasing hot water from the engine to the radiator to be cooled by the fan then close off so the water can cool in the radiator. if this cycle doesnt work properly then the thremostat is usually the culprit.
replace as nessesary.
good luck