I am meaning after you changed the oil and got it running, not by the customer. Typically the customer won't drive it hard enough. If glowplugs, batteries, starter and ICP voltage all checks, go bag the piss out of it.
I only suggest this because we had in 3 trucks in the past 2 weeks from an aircraft fuelling company that sat for a long time and this is what we had to do to get them to start normal.