Used to ask my friendly garage if they would give me 5 litres out of their big drum of oil for a tenner. I had to pump it out obviously, that was in the days before everyone went all officious and now refuse you to even walk about in their workshops for "insurance purposes" (so you can't watch them poking holes in your car at MOT time)!
Every car in the uk used to run on garage big oil drum oil. That was in the days when tolerances were measured in inches not nanometres.
All but one of the oil changes I did on my Omega used semi-synthetic 10w40 out of the 200litre drum at work. All my other cars got the same stuff too, including the Avenger, 2.9 Capri, 525, Legnum, Morris 1300 and anything else that came my way. Before that, I used whatever we ordered for the fleet of Transits.
There needs to be a good reason to obsess about oil beyond the suggested grades, and most engines don't justify it. I certainly wouldn't be carting several cans of an odd grade back from Spain just because it was cheap....