on a E28 BMW M525i is the round thing that bolts intothe bottom of the...

[LewisAtherton]

...sump the oil pressure sender? i have posted this question for my step dad he has bought the car recently and he has found a problem he is wonder is the round thing that bolts into the bottom of the sump the oil pressure sender

 

[RobertM]

No that's the sump plug used to drain the oil, the oil sender will be on the engine block.

 

[BROD]

That is the oil level sensor
The oil pressure sensor is beside/near the oil filter assembly.