I buy clearance oil or oil that has a really good rebate going on. I have around 50 gallons of Delvac1300 15w40 on hand and yes I got it cheap. 1, 2.5, and 5 gallon containers. I have around 10 gallons of Pennzoil 10w30, 10w40 in quarts on hand and yes I got it really silly cheap on a bulk buy. I have quite a few containers of Mobil1 in various weights that again I picked up really cheap. Other brands of motor oils, gear grease, grease, and 2 stroke oil. I keep my eyes on BITOG and yes it has paid off many times. I use the oil for farm equipment, vehicles, and small engines. I have a good reserve on hand because I need it. Syn or Dino, all get used. I repair/service commercial mowers as a side business. During mowing season, I take at least 1- 5 gallon can of oil weekly to a friend who heats his garage with an oil fired heater. My experience is any API rated oil works good and does the job. Keep the dipstick on the full mark and the engine doesn't care what brand it is.