If you're going to change it every 6k, then I don't think it matters what you use for oil. But if you want to go significantly beyond that, I'd recommend sending your oil off to
Blackstone Labs to see how much TBN you've got left.
I run Mobil 1 Extended Mileage and they say 10k is not a problem, try 12k.
Different oils have different amounts of additives, the TBN is what is the "anti-wear" part. The extended mileage stuff from Mobil 1 has a lot of that, which is then used up over time. Blackstone labs measures that after xx miles, and predicts how much further you could have gone. Each manufacturer uses a different anti-wear blend of additives so Mobil 1's isn't necessarily the same as Amsoil's.
If it isn't putting a lot of metal or fuel into the oil, and as long as it maintains it's viscosity and flashpoint, you can run oils until the TBN is used up. That's what Blackstone labs reports will tell you.
Of all the engines I've owned, the FJR is wearing the best. We do the blackstone labs for all our vehicles every 3-4 oil changes, and here's what they with the mileages listed:
FJR (82k): 10k - 12k
VFR (100k): 4k
Prelude (128k): 4k
Audi (105k): 5k (burned oil, sold, good riddance)
Subaru (118k): 8k
SV650 (32k) : 8k
EDITED to fix the link (thanks Mountain!)