I buy what is a "Great Deal" on oil when I find the deals. Since, I've been doing some Oil changes for Family, Friends and Neighbors....I don't wait until the last minute to buy oil. I look for closeout specials, markdowns and great sales and store what I can.
Thanks to a poster on BITOG.....I recently picked up 3 Cases of Valvoline WB, 5W-20 on a Amazon/Valvoline deal. 4 Gallons per case of VWB for $21 a case, before the rebate and Free Shipping. I have one case left and I'll be mixing it in the future with another Valvoline product....most likely Durablend or Valvoline Super Blue Diesel Dino....if I find it on sale.
So, it's all relative to your situation if you buy at the last minute or take advantage of sales, computer glitches, errors by retailers or closeouts.
In my case....I look for sales or the generosity of other BITOG members letting us know about specials, closeouts and sales.