To be certain, you'll need to:
1. Consult your owner's manual: This is the most reliable source. It will have a diagram showing the location.
2. Look up a repair manual specific to your year and engine: Chilton or Haynes manuals will have detailed diagrams and explanations.
3. Use an online parts diagram: Many auto parts websites (like AutoZone, Advance Auto Parts, Napa Auto Parts) allow you to search by year, make, model, and engine to see diagrams of your engine. Look for "oil pressure switch" in the parts diagram.
General area: It's usually near the front of the engine, somewhat low down, and relatively accessible without removing too many components. But again, the precise location isn't standard across all G20 models.