OKEYDOKEY...you heard it here first...I'm not going to patent it, so it's now open source.
Shannow's better version of this Castrol Gimmick (and it's a gimmick, as the storage containers are tiny compared to a shop's needs).
Make an oil station, like the paint blending station at the hardware shop.
Computer based recipe book...6 basestocks (3 syn viscosities, 3 dino, 4, 6, 8Cst), a couple of VM packages, DI packages. Basestocks in 1,000L bulk boxes in store room (or pressurised beer kegs behind the station)
Car needs syn 0W20...x parts 6Cst, y parts 4 Cst, two pumps of VM(a), 5 pumps of DI.
Car needs sem syn 5W30 x parts 8Cst Dino, y parts 4Cst Syn, three pumps VM, 6 pumps DI.
Couple of other additive options (Lucas, LOL, Moly, Ti).
Would genuinely reduce inventory and waste.
and I'd have one in my garage, and never leave it's side...