Toyota Repair: 95 Toyota Camry Oil leak problem, engine oil pan, causes of oil leaks

Question
What are some of the common causes of oil leaks.  When I open the hood (2.8 liter, 4 cyl.) looks like it's leaking from the left, back side of the engine.  I look under the car and I see the part that it's dripping from but don't know how to identify the part.  What is this part? that I need to change?

Answer
There are two possible causes for this oil leak at the rear of the engine, one is that the engine oil pan may be leaking or the engine rear main oil seal,I would recommend having an flourescent dye added to the engine oil to pinpoint the leak, a rear engine main seal replacement is very expensive( the transmission has to be removed) I would also recommend having an expert technician visually check it after adding the dye, the leak is located using an infrared light.