I have a 2006 GMC Sierra with 215, 000 miles. It has a 5.3L with 4L60 (M30) transmission. I essentially want to do a "spill and fill" service to it. Just drop and clean the pan, replace the filter and top off the ATF. I don't know what type of ATF I should replace it with, and didn't have a service history when I bought it. What should I use? Synthetic or conventional? What type should I use Dexron 3 or Dexron 6?