The oil warning light on my 2.5 tdi 2001 T4 comes on now and again at slower speeds after a long run. It may be a coincidence but the problem seemed to start after the last oil change. I've asked the garage and they told me that the oil was 15W-30. The van has done 190,000 miles and I'm if wondering if 15w-40 or even 15W-50 would be better considering that the VW 505 specification goes up to 20W-50. Another thing that has occured to me is that the oil might be getting too thin because the oil cooler isn't working properly. Is this a possibility?