09 Chevy Cobalt - My cobalt has a digital read mileage, mpg, etc. Should I trust the digital oil % reading for getting my oil change? I've heard it is set at the manufacturer for miles not viscocity.

It is set for neither. It is computer controlled. Data on temperature, speed, miles, RPM, etc. etc. is entered in and calculated on the fly and that is what show up in the reading. You maybe sure many engineering man hours and probably millions of dollars went into developing that system so it would give accurate information. GM warranties those engines on the basis of oil changes being done when the computer system says they should be done. For that reason alone you can be sure the computer is calibrated to err on the side of caution. As long as you are using the recommended oil, even if its expensive, you can follow what the computer tells you to do and rest easy. Pay no attention to the old school guys that still think they are somehow doing their car a favor by changing oil every 3000 miles, rain or shine. All they are doing is wasting money and oil. If you deviate from the recommended oil like I have with my '03 SS then you need to do your homework and possibly adjust your oil change interval accordingly. A cheap out of spec oil will require more frequent changes, a higher performance oil, if there is one for an '09 will allow for an extended time. I run my oil through two complete cycles of the car's computer with an oil change at the half way point. My decision to do that is based on hard published testing results, not here say and speculation. Hope this helps. They're great little cars. Take care of it and it will take care of you.