i want to leave original filter in place as well so do i go with a 30 micron or 5 micron filter or some thing differant
The determining factor is probably where the primer pump is located.
I am not familiar at all with the Isuzus, as I have a Fuso, but on my truck the primer pump is at the engine, so it has to suck the fuel from the tank. If there is too much resistance in the fuel line then this will limit the fuel getting to the high pressure pump, which would not be good. On some trucks the primer pump is in the tank, so it pushes the fuel to the engine. Pushing fuel is much better than pulling when it comes to adding additional filtration.
These trucks are known for running hundreds of thousands of kilometres, so I figure that the OEM fuel filtration must be pretty good already, so personally I saw no real need to improve it.
The OEM fuel filter on my Fuso has an electronic water sensor, but the casing is solid black plastic, so there is no way you can do a visual inspection for water in the fuel, which is something I did not like. So, my main reason for adding the Racor filter was not to improve filtration; it was so that I had a water trap in front of the OEM fuel filter that I could inspect easily.
Before adding my Racor filter I temporarily installed a vacuum gauge in the pickup line as close to the engine as I could to test the vacuum at that point. After adding the Racor filter there was a negligible increase in vacuum, which gave me confidence that I would not be affecting fuel flow with the addition of this filter. I am using 30 micron filters in the Racor, as this reduces the possibility of fuel flow restriction if it gets a bit dirty. I also have a permanently mounted vacuum gauge on the Racor now, so I can see if it does get dirty.
So my advice... don't assume anything, test your setup before and after so you know for sure you are not fixing one problem while creating another.