Originally Posted by RichieST

I don't think these have an oil cooler? Certainly don't remember my old mk1 1.6 having one.

Has you topped up the coolant at all? Did you use the right stuff? If you use different types then they can react and sludge up the cooling system. As you say, I'd suggest draining the coolant and oil and refilling with new then keep an eye on it.