I use a genuine paper filter or the equivalent from RYCO or REPCO.
onebob
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+1 for checking for oil in the ECU
Cheers
Tony
Assuming it is a TD5, a very simple thing that makes a large difference is cleaning the MAP sensor. It is in the top of the intake manifold. Held in by 2 8mm bolts.
I clean mine with de-greaser. For the little effort it is worth trying.