oil sending unit

thought I would get this under a differnet topic . Talked to parts and service of a Maxda dealer and was told that a 2002 sending unit will not work , and that my sending unit by it being big was changed out to make the gauge a real guage .
I gues that I need a sending unit that is small but still is for a guage or a big sending unit that makes the guage a real oil pressure guage
If any of you can clear this mass confusion up I would really appreciate it

The sending unit I have is pretty big and I have a feeling it is bad but not sure how to test it to know what to get