Newbie - Question about loading wood furnace

Hello, newbie here with new U.S. Stove 1602R as of a week ago. I'm using it as a stand alone unit to supplement propane central heat source during central Texas cold fronts. So far I am thrilled essentially eliminating propane use during 25 degree front earlier this week. However, I need help trying to get at least 6 hours of effective heat. The fire box capacity is 160 lbs of coal or equivalent 28" wood. I am burning dense dry red oak, but I have only loaded it with maybe 40 lbs so far. At first I was getting quick high temp burns lasting maybe 2 hours. After shutting down all controllable air sources it extended heat to about 3 1/2 hours. Everything I've seen says I should get at least 6 hours out of it. Here is my question: If I load it more heavily, say 60 or more lbs of wood, will the heat reach a limited constant based on available oxygen and therefore burn much longer? I want to avoid loading it heavily and having a bad overfiring event that I can't control. I assume the limited oxygen the fire is able to suck will keep everything under control regardless of the available wood in the fire box. Is this correct. Thanks for any info or advice.

Leo
Austin, Tx

I know nothing about your stove, but I would love to only have to weather a 25F front. Our fronts hit the -40F range with howling NW winds straight from the arctic. Hope you get a better answer, and stay warm.