As far as what the gauge measures, oil gauges measure pressure downstream of the pump. (I.E. oil ascending unit if I'm correct) One possibility is that the diesel additive caused the engine to run warmer than usual and the increased temperature dropped the oil visocisity. To test this you'd need to run a tank doing you normal driving with and without the additive. Was it still in the "Ok" area? Have you changed your oil lately? We're you in higher altitudes or warmer climates? Thinner air will make the engine run warmer as well as the climb to get there.

Frank.

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'82 300SD