Cat c 15 fuel filter question

Hello I would like to ask to be enlightened on the fuel filter system on a cat c15 sdp.

Cat mechanic was telling me that the primary fuel filter needs to be changed every 5k miles to protect emissions components and secondary every 15k.

He went to say that truck idling the fuel is filtered through the secondary and once truck is running it's filtered through the primary. This makes no sense to me and he couldn't really explain his reasoning

 

Thank you very much that makes perfect sense. Do you know what the fuel pressure spec is on these sdp?

 

Thank I would really appreciate it. Driver had to take the truck to the cat dealer because it lost power. The mechanic checked pressure and it was supposedly

 

220 psi and according to him it's needs to be in the low 300s. They changed the pump and both filters and truck is running good again. I'm familiar with the earlier acerts and 3406e and when I messed with the pump I knew 110 was the max or else I was risking the injector orings starting to leak from the added pressure