While some types of grey literature are commonly included in systematic searching, such as conference abstracts and theses, to some extent the type of grey literature you need depends on your question and the evidence required to answer it.
For example, randomised controlled trials are typically used for questions focused on the effectiveness of specific interventions. For this type of question it would be important to include trials registers in your sources.
If instead you were investigating user perceptions of a particular service, a qualitative design incorporating interviews or a questionnaire might be more appropriate. The study might never have been published, but mentioned in a report available via the organisation's website.
You should carefully consider the types of grey literature you will and will not include.
The following video summarises some common types.
Video Length: 5:13
|conference abstracts||conference papers||patents||statistics|
|blogs||emails||preprints||theses and dissertations|
|clinical trial register entries||informal communications||repository content||webpages|
|practice guidelines||archival material||standards||wiki articles|
These sections of the Cochrane Handbook recommend specific types of grey literature to be searched for compliant systematic reviews.