After nearly 6/7 months (3000 miles), the oil in my 355 was still very similar to its original condition i.e. very clear and runny - I was struggling to get a true dipstick reading. But within the space of a month it has now turned very dark (black) and is quite a bit thicker. Is this normal for the oil to change condition like this in this space of time ? It's due its annual service in a couple of months (1000 miles) should I change the oil now or wait until the service ?