I just checked the oil in my TA (4500 miles up) and noticed the dipstick has become quite discoloured since last check about a month ago. It was normally a clean white / grey colour but is now a sort of dirty oily brown colour along it's entire length but cleaner at the bottom where immersed in oil. Anyone else notice this and is it just normal discolouration? Seemed to appear over about 1000 miles.