Coolant Question

[raleigh753]

I have a 2013 XC70 D5 215bhp and an issue with the cooling system. Driving every day in to town shopping etc the 'stop vehicle low coolant' message comes on after a few days. When checking the coolant is below the sensor level , hence the alarm. When the engine is cold the next morning I open the coolant cap and there is a rush of air and the level goes up to normal . There is no mayonnaise or combustion fumes in the coolant , nor is there air bubbles with the cap off and the engine running. I would like to know what type of coolant should be in the system , blue or the red OAT (organic acid technology) type . I have searched the internet and found no definitive answer and the coolant currently in the system is a sludgy brown which looks like its got Radweld or something similar. I want to drain , flush and refill the system and try and determine the issue causing coolant system pressurisation and obviously need to use the correct type of coolant . I have already renewed the coolant cap . Incidentally the warning message came on the other morning and after giving the car some beans the message disappeared and on checking the coolant level was correct so I suspect a blockage somewhere caused by the additive .