Temperature Warning Light

Hi everyone, I have a Mk3 golf TD that has done around 160k miles. It has always been very reliable but recently the temperature warning light started to come on after about 40 mins of driving even though the temp guage wasn't indicating a high temperature. A quick feel of the radiator hose confirmed that the car wasn't overheating and that the temp guage is working correctly. I checked the fuse for the fan motor and that is ok. Then I checked the coolant level. The coolant level was good but I noticed there was all this oily gunk in the expansion chamber. I had a word with the lads at work and one of them said he had a similar problem with a 1.6 TDI Passat that he used to have. He said that there is a water cooled oil cooler on the diesels and it is quite common for the vanes to break down inside it releasing oil into the water system which then fouls the temp sensor putting on the warning light. Is this correct? has anybody else experienced this? How did you fix it? any help would be gratefully recieved :D

I used to have a mk3 golf GTD and it had the same problem, although I didn't get the temperature warning light, I did get a mass of oily gunk in the coolant expansion tank, the oil cooler has a water jacket on the top of the oil filter housing and the coolant chamber has probably split letting oil into the coolant circuit.
The only thing to do is to replace the oil filter housing and coolant hoses and drain and refill with fresh coolant.