Most OM606 engines are in W210 cars, which do not have oil pressure gauges, thus data is limited to the 1995 E300D (W124/OM606). As long as the pressure gauge always pins high above about 1300 RPM (I forget the factory number) you are OK. Gasser oil's big disadvantage in a diesel is its limited ability to absorb soot. Depending on how often the PO changed the oil no harm may have been done.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970