1997 Other Opel Models Crud under the oil fill cap

My Daughter’s Opel Cosa 1997 1.2 liter 4 cylinder and about 70,000 miles has cream-colored crud under the oil cap and on the top of the dipstick tube thru the winter. I put in some “Bars Leaks" and it was better, but the weather here in Europe was also warmer. We bought this car used and according to the records the timing belt was replaced, but I can’t tell if they also replaced the water pump and thermostat as recommended, but also is an added expense. The timing belt drives the water pump and the thermostat is behind the water pump. The compression check showed all 4 cylinders at between 190 – 195 psi after pouring in the Bars Leaks, prior to that I don’t know. It uses a small amount of oil, maybe a quarter quart in 3,000 miles. It doesn’t use any coolant. No check engine light and otherwise it runs fine, but according to the gauge it takes longer to warm up and runs a bit cooler then our other Opel Cosa. Could that cream colored build-up be caused only by the thermostat not allowing the motor to get warm or does it have to mean a coolant leak into the motor by the head gasket or a cracked block or head? Since the warmer weather I don’t see the crud anymore and the car is driven about 20 miles every day on country roads at somewhat around 60 MPH.