Suggested PMS for our L300 Fleet 4D56

Hi Tsikot Community. I would like to ask for your help, i have read forums regarding maintenance on fellow 4D56 equipped vehicles and became interested on uniforming the PMS routine for my 11 fleet of L300 for my business.

Currently, the driver of the trucks are left with the responsibility of keeping their machines in tip-top condition. But recently, two engines died on us in the period of two months which made me think the lack of proper maintenance caused these. Maybe the driver or stressing the engines on longer durations of oil and filter changes, or worse, charging us for a PMS which don't actually happen (I happen to catch one of my employees at an instance).

I would like to know from you what will you suggest on maintenance as I am getting various outputs from CASA and Mechanics. CASA is suggesting every 5K, Mechanics are suggesting every 15K.

Here are the details:
out of 11, 3 units are within 1 year and are CASA maintained for the first 10K.
every unit is travelling at night so expect pure Highway conditions most of the time.
evey unit clocks around 40000 kms a year.
OEM or aftermarket parts? and would it be better to use KM reading for oil changes or periods like quarterly oil change?

Many Thanks! :)