Try putting less wood in, and maybe splitting it more. If you have a good base of coals going, you shouldn't have any problems. Don't let the ash get too thick. I am on my second season with the same stove and have no complaints. I find it burns better with more of the firebrick exposed. Also, I'd move the diff to 10 instead of 5, at least once you get the coal bed set. Empty out your ashes once a week in the ash cleanout. It helps to keep the air flow. I find with keeping a smaller ash bed, you burn the coals more into ash that can fall between the firebricks.
Could also try loading in some kindling with the oak.
I'm in the U.P. of Michigan and this year I'm getting 12-16 hour burn times on smaller loads of wood than I burned last year. Granted, last year was much colder with multiple sub-zero nights, but on comparable nights, I'm doing better this year than last year, where last year I let the ash bed completely cover the firebricks. I burn mostly sugar maple with some white oak and white birch, plus some occasional softwood.