1991 350SDL diesel has ok oil pressure when first starting cold. After about two to three minutes it drops at idle to close to zero on the gauge and will go to just beyond "2" on the gauge at 2000 RPM.

MB dealer here says to determine if it is oil pump or bearing failure they would dump the oil thats in it and refill with 20-50W, then if pressure holds steady it would be a bearing failure if it drops as it is doing now they would say it is an oil pump failure. Does this sound like an accurate way to make this diagnosis?

Thank you