Oil pressure is at it's worst when the oil is hot and the engine is at idle. It is also very much affected by the viscosity of the oil being used. Do you know what viscosity oil you are using? I would guess you should probably be using a 15w40 diesel oil but a 20w50 diesel oil would give a higher idle oil pressure. The stresses at idle are low so 0.5 bar is fine but 0.3 sounds a bit low. The number after the w in an oil viscosity rating is the rating at operating temperature. Most old diesels require a 40 but a 50 is thicker so will give a higher idle pressure. If you are using say a 30, try a 40 and a new filter before you decide you have a problem.

Diesel oil is black soon after an oil change so if you want to be sure it was changed you must look at the stick before you drive home.

It is normal to keep the coolant down a couple inches in a radiator IF you do not have a overflow tank. If you do have such a tank it should keep the radiator full.

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'97 E 300 D