First, check the oil level. If you have oil, most likely something with the oil pump is damaged. If you have no oil, well then obviously you developed a massive leak somewhere.

If it was just the oil sender going bad, why would your engine be running rough all of a sudden? Sorry to be the bearer of bad news, but the fact that the engine was running rough along with the low pressure to me points to the fact that it is engine related versus gauge related.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD