Haven't looked at the brand of oil for years. Buy it when I need it along with the other parts, as I don't want the rest getting in the way over the next three years. That's worth more to me than a possible few pence a litre saving.
Yes, I agree its annoying having to by 20l drums. But its half the cost, and I kept 3 of the 5l containers, so the 20l gets opened for the first oil change, then the rest is decanted into the empty 5l cans. Now had I been clever enough to keep 4 5l cans, I could decant straight away.
20l of semi lasts us 6-12 months, depending on her mileage (which varies dramatically). Typically the Zafira gets an annual change, as it covers hardly any miles, and the Omega gets oil every 5-7k, depending on the weather outside, and how much petrol running she has been doing.