I didn't drill out the adapter fitting to the same size ID -8 because it wouldn't leave very much material on the -6 side of the adapter. I've put 500 miles on the bike and rode continues for over 100 miles and didn't have any problems. I really don't think the oil flow rate through the cooler is fast enough for the adapter to cause any major restrictions.
I used teflon tape on the -6 thread of the adapter fitting because the oil cooler is not meant for a AN fitting (the threads on the cooler are metric) but the threads are close enough to match up with the thread pitch of a -6 AN. If I had some type of metric fitting with a groove meant to seat the O-ring perfectly I wouldn't use Teflon tape and that's why I didn't use teflon tape in between the adapter and elbow.
I've built countless number of automated machines and I used teflon tape on most pneumatic fittings with out a problem. Would I have a problem if I didn't used teflon tape? I don't know but I haven't had a problem yet so why change it. I know of to apply teflon tape correctly and that can be why I haven't ran into an problems (yes, you sure can apply teflon tape wrong). I think teflon paste sucks. Stuff makes a mess and it never seems to dry.
At the end of the day I don't have any oil leaks, bike hasn't grenaded yet, and I think it look cool. Do you need to use teflon tape, probably not but you can risk the chance of having oil leaks.