Oil in a diesel won't look clean for very long.
You probably have alot of deposits in there and build up.
The new oil is picking up the black oil on the way down to the
Short trips with the truck wouldn't let the oil reach a hot enough
tempature and would cause build up.
Waiting to long to change the oil would cause build up.
Lots of idling would add to it.
Flushing is probably just washing alittle bit of it away.
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You could keep flushing it but that could end up blocking
up your oil passages.
Take off the valve covers and see what it looks like, if it is
good and caked up then that is a good indication of what the
rest of the engine looks like.
Manually cleaning it would be the next step, just cleaning out the
caked up sludge on the heads would help out alot.