Hi Mohammed,
I have a gs300 mk 1 had a very simular problem, was topping up the expansion tank with water every 4-5 days. The heater was always cold too.
I had a coolant change done and it cured all the water problems. The cause was an air lock in the heating system. At the back of the engine there is a water hose coming out of the car it also has your temperature valve on it, check this works when you alter the temp control on the dash, also Gs300's can be hard to bleed because this hose is higher than the radiator. although yours sounds like a head gasket with the oily water you can easily check these other points. Hope this helps ;)
Jon