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I know you can buy it in 2.5 gal jugs,usually two to a case. After that I think it goes to 55 gal drums and then bulk dispensing systems in 500 gal tanks or larger from what I've seen.
Just watch the flyers for your local auto store. Oil often goes on sale for 25%-40% off. The sales will rarely line up with when you are due and wanting to do the change. Therefore, buy and build your stock whenever the stuff is on sale, anytime.
Back in the old days. ( the 80's), our shop had 5 vehicles and also 4 personal vehicles of the family. we were able to by Valvoline 10-40 in a barrel that I think was a little smaller than a 55 gal drum. we had to purchase a hand pump that threaded in to the drum. but it sure did drop the cost of an oil change. Im sure you can still purchase oil by the drum. a lot of lube places seem to still have them.