I feel like you can sustain a reasonably active military with 50 to 100 oil rigs. I don't see a need to ever buy, unless you're going really small (in terms of acreage).
As you grow, your level of tech will fluctuate - even that small 20 acres you get a day could cause that 81.66 to jump to 82.01 or something. At least that's my understanding...