To be certain, you should:
1. Consult a repair manual: A Haynes or Chilton manual specific to your 1988 Chevy S10 and engine will show a diagram and precise location.
2. Look at a diagram online: Search for "1988 Chevy S10 oil pressure sending unit location [your engine size]" on Google Images. You might find a picture of the engine bay showing its location.
Remember to be cautious when working near the engine; it might be hot. Disconnect the negative battery terminal before starting any work.