error filtering title eq 'newsletter'

I'm having trouble filtering leads using the `$title` field. When I use the following query:
https://{{server}}/sdata/{{install}}j/sagecrm2/-/Lead?startindex=1&count=10&where=$title eq 'newsletter'

It shows all the leads instead of filtering. Then I attempted to modify the query to:
https://{{server}}/sdata/{{install}}j/sagecrm2/-/Lead?startindex=1&count=10&where=title eq 'newsletter'

But encountered an error message:

    "$diagnoses": [
            "$severity": "Error",
            "$applicationCode": "sdataXXXXX",
            "$message": "BadRequest ",
            "$stackTrace": "sdataXXXXX",
            "status": "BAD_REQUEST",
            "$sdataCode": "ApplicationDiagnosis"

Could someone guide me on how to correctly filter through the `$title` field?