oil will have little to no effect on E-test . weak or miss-firing spark plugs,weak o2 sensors, vaccum leak, cats are biggest problem. A little story for you 1998 mercury mystique with weak 02 sensor.Go in for etest at ford dealer tech comes back man your lucky just squeezed through the test. HC PPM reading 67 limit 67 , CO reading 0.12 limit 0.37 , NO PPM reading 14 limit 501. So i said great and took off, about 4 miles down the highway you guest it the check engine light comes on stop at my local mechanic pulls a code 02 sensor weak. And really the car was runnig great but rich enough to throw a code and really should have failed the etest. timing was on my side. My next e-test 2 years later with new 02 sensors, the the HC reading went from 67 to 11 just shows how much a weak or bad sensor can change a reading.
