Supposed to be fully synthetic.

BUT ... it's far more important to change the oil and filter every 6-10k miles. Also, when draining the old oil out, use the sump plug, not one of those silly pumps that haul the old oil out through the dipstick pipe

Since my 2006 Fiesta Tdci passed 100K miles and is now on 185K, I took the decision to use a fully synthetic 5w/40 with an A3/B3 or B4 spec., rather than the 'recommended' 5w/30. I have used Mobil 1 0w/40 through a particularly cold winter, on the idea that a '0w' rating would be better for starting on an icy morning, although Comma 'Syner-D' 5w/40 which I normally use in winter causes no problems - it always starts first turn. Last month, Euro Car Parts were offering a 5w/40 (5L) for less than fifteen quid, and you got a 'free' oil filter with it, so the Fiesta has that in it now, and at that low price, I'll give it an extra oil change in another month or so. Whether I'm right or wrong, the thing keeps going. It's had its problems over the years, but its always got me home.

For clean running, try the often-mentioned 200:1 shot of two-stroke oil in your diesel (that, in the Fiesta, is 150ml of two-stroke oil in the tank when filling up from the yellow pump light coming on). You won't get 2T cheaper than Morrisons - £2 for 500ml bottle or £4 a litre.