Obviously, I would have no way of knowing the exact answer, but will make a few comments and you take as you wish.
Here is what I understand:
A 2003 with 10k miles
Poorly maintained
Using 1 quart per 700 miles
I suspect that the engine is quite dirty inside and there is a good chance the top end of the cylinders are in need of cleaning and possibly even a few piston rings not moving freely to seal the combustion chamber.
If it were mine I would install some very high detergent oil (I think Shell makes some for diesel engines) and use that for 1000 miles, change it and repeat, changing the filter each time. I would do this maybe three times. Also, and at the same time would use a top end cleaner. GM dealers sell a top end engine cleaner. Each time you change the oil, take it out and run it for 45 minutes or so at highway speeds.
Perhaps others will chime in.
Here is what I understand:
A 2003 with 10k miles
Poorly maintained
Using 1 quart per 700 miles
I suspect that the engine is quite dirty inside and there is a good chance the top end of the cylinders are in need of cleaning and possibly even a few piston rings not moving freely to seal the combustion chamber.
If it were mine I would install some very high detergent oil (I think Shell makes some for diesel engines) and use that for 1000 miles, change it and repeat, changing the filter each time. I would do this maybe three times. Also, and at the same time would use a top end cleaner. GM dealers sell a top end engine cleaner. Each time you change the oil, take it out and run it for 45 minutes or so at highway speeds.
Perhaps others will chime in.