The most damaging particle in a diesel engine's fuel supply is ~20µM .... and almost all engine mounted 'guard' filters are in the range of 15-20µM. This filter is mounted on the engine as a 'last chance' filter just in case an upstream Racor, etc. system fails either by extrusion of soft particles or by pleat rupture.

The typical 30µM Racor prolongs the service life of the next in series 10µM by acting as its 'prefilter'. The 'workhorse' filter is the 10µM.
A 2µM will require 5X the work (pressure drop) to do the same job as a 10µM and will put a large and quite unnecessary ~5 times greater load onto the lift pump's diaphragm.... and thats for a 'clean'/new 2µM of the SAME surface area of the preceding 10µM ... and the 'guard' is typically 1/3 the surface area of the typical Racor 10µM. It takes 'work' to do filtration, so dont unnecessarily overload your system with unnecessary work ... and then increase the vulnerability to a premature lift pump diaphragm failure.

And worst of all, all these filters are only 'nominally' rated, meaning that they only capture a high proportion of particles at their 'rating', not ALL at that µM rating. That engine mounted guard filter being with a comparatively small surface area (high velocity through the filter media) will quickly 'choke' with particles much smaller that its 'rating' and rapidly shut down (choke) the whole flow. A filtration engineer could describe that engine mounted 'guard' filter as a 'fuse', to guard against and then quickly shut down to prevent undue particle overload coming from a failed upstream filter set.

Typical filtration 'train' for a diesel engine: tank--> 30µM (if the tank is historically easily fouled) ---> 10µM ---> 15µM (guard filter) ---> engine. Unless your engine specs. state otherwise.