Took me about 800 miles on the WRX and GTI combined ... but finally beat this damned 8-6 challenge. Every race, I was either taken out, or I made a mistake (like, I was in first place and hit "no refuel" by accident, or I didn't stop the refuel and wasted several seconds going to full). I tried different methods (no refuel with map to 6 and lots of blocking, pitting on 9, pitting on 12), different cars (GTI, where I pitted on 9, but then ran out of fuel when I was in first place on the final lap) ...
Finally, I did WRX, map 1, RS front RM rear, pit on 12, fuel to just over 10 laps. I was in top three (maybe first or second?) when I pitted, then climbed back. Several times I did this and could not get back to first, because I had no help drafting and the other WRX was in first, and I just couldn't catch it.
On my final time through, just like previous ones, I was in first and got taken out ... but this time, I miraculously went sideways and then got it straight in time to block the Viper on the outside, giving me enough of a cushion to get back in front, and finish the final lap ahead, flipping off that jerk in my rearview mirror, my hands gripping the TS300 wheel as tight as I could, as I called that WRX and Viper every name I could think of.
(All I have left for all-gold completion is the Ring in both circuit experiences and the 7-* challenges.)