This is the OEM Subaru fill plug for the rear differential on many late-model Subarus.
Plug appearance may differ from photo.
Fits all 2002-2019 Subaru WRX. The early-model WRX plug also fits most Impreza, Forester, and Legacy models from the 1990’s through 2017 or so. Note that the WRX STI does not have a fill plug for the rear diff; you fill the diff through the rear diff temp sensor hole.
