Um... politely speaking, the vacuum gauge is a tried and true indicator for the equipment operator, and a good investment if your diesel is high value (trucks have them on the dash). Then call the filter manufacturer and talk to the folks that designed the thing, not the salesman. They've already spent the coin to do the testing and know at what differential pressure the filter should be changed. This all assumes the filter doesn't leak or get crapped up with oil (not an issue with daBenz's oil bath filter).

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daBenz - 1970 220D