Running coolant without the additive will in time cause pin-holes in the cylinder sleeves in a diesel engine, its called cylinder wall cavitation or liner pitting. Regular anti-freeze does not have all the stuff in it needed to prevent cavitation in diesels, you have to buy an off the shelf additive like the Motorcraft or Fleetguard stuff and use test strips to check for proper charge. Some high-end diesel anti-freeze has the additive already in it.
Witnessed a cylinder sleeve pulled from a tractor the other day that was all pitted and full of pin holes from cavitation, so I'm looking to get my coolant in order in both my trucks soon.
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'96 3500 Ext-Cab Dually, 2WD, 5-spd, 370's, #8 plate, Southbend Con OFE, 4" turbo back exhaust, BHAF, 3.54's, 235/85/16's, 110K miles