Buy fresh BB for each of your batches if you go more than 6 months.
Basically BB is used to reduce viscosity of the oil. That's why I use Mig840(a fully saturated highly refined oil with a very low viscosity) and brew with low concentrations like 100mg/ml. I add 1% Benzyl Alcohol and try my best to avoid using BB because the shit is nasty.
Try tasting some BB to see what I'm talking about, you're injecting that garbage into your body. Years ago I screwed up a batch of TNE by putting 30% BB in it, made me feel weird when I injected it, like I was getting sick and run down everytime I injected, felt like I could taste that nasty BB in my mouth. I pitched the batch, brewed at lower concentration and left the BB out and it was great.
My motto is simple, oils good, solvents bad.