Which Oil Is Best?

[Major Rant]

I take it to my council recycling centre so that it can be turned into Asda 10/40.

If you did, it wouldn't meet the SL or CF spec - so you wouldn't want to buy it, would you? :pokefun
Having said that, there are oils out there (notably Car-lube 20/50) that don't meet the spec and look suspiciously like they ARE made from waste oil.
I would use them in my Mk1 Escort (RIP) or my lawnmower.
(Amusingly, Briggs and Stratton sternly warn you against using any oil other than special SAE30 Lawnmower oil in their 1940 side-valve 1000RPM
agricultural wonder. Available for only £10 a litre at your local Wyevale in a green bottle :rollin
Does the message sound familiar?)

For an oil to meet that spec it has to contain enough zinc-dialkyl-phosphate to act as a "barrier of last resort" against metal-metal contact when the oil film breaks down cos its too thin to cover the impact forces as your piston ring rattles down the bore 150 times a second. That sliding-seal in the presence of hot exploding gas is a far more severe test of oils than the nice clean push and slide cog-cog contact in a gearbox. I would be more worried about my gears wearing out after exposure to all those carbon and nitride particles that contaminate the oil on the barrel walls.