I have a Cerato 1.6CRDi and about 3 months ago changed the engine oil myself to MOTUL ECO-Energy 5W-30 synthetic oil and since then have recorded a significant increase in fuel economy. I used to get and average of 47mpg and now get 51mpg over the last 3 months. I know the warmer weather probably helps but being a bit anal I have the numbers on a spreadsheet from last summer that still averages 47mpg over the same period. I now have over 70k miles on the clock. I don't know what type of oil was in it before as I had it serviced previously by a local garage. Changing the oil and filter on this car is very easy. My driving is mostly on dual carriageway.