Your OLM is calibrated to dino and quite conservative. I assume you have a OHC engine, and that the 3.8 has been dropped, right? I have a Malibu Maxx with the 3.5 ohv and I average about 7-8,000 miles on dino before the OLM gets down to 10%. Not sure how the other engines compare but GM is usually an easy engine on oil.
Conoco/Phillips/Ford/76/Kendall (did I hit all of 'em?) has a lot of group III because of Conoco/Phillips refining capacity. Used to be able to pick up Phillips for about $1.70 at Wal-Mart but they dropped it. I haven't looked at their oils closely lately, but I think they all may be labeled as semi-syn.
I generally go with Pennzoil 5w30. It's a very, very good dino oil with lots of group II+ basestocks and a very good ad. pack.
If you used a group IV synthetic or even a top end group III, I would think you could easily extend your OLM change cycle. I'd get nervous, however, driving with the indicator at zero. Dropping a group IV at the full change cycle might be a waste of money but would certainly give you extra protection. Again, that OLM is very very conservative so not to worry running it out, even with dino.
Although Valvoline has a reputation for being a rather weak dino, I've also heard very good things about SynPower. Pennzoil Platinum too if you just gotta have synthetic.