Oil pressure sending unit for '87 300SDL

When I turn the ignition on, my oil pressure guage reads maximum pressure and stays there. My mechanic says that the sending unit may need to be replaced but since my engine is sound and there is a backup
warning light, it isn't absolutely necessary to replace it.
I'm thinking that I may want to replace it myself if I can locate a service manual. What is the best sourced for parts for this car? Can anyone provide me with this repair procedure?
Thanks all!
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I have same car and have same reading. This happend after i changed seals on the oil filter stem.

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