[AndyV7o]

As for other stuff, im not really sure where to begin so Ill just start.

A5/B5 oils, there are seemingly plenty, in original grades (usually 0w30) and alternatives, so just refer to the above!

Older engines from early 2000's back to late '80's/early '90's tend to require A2/A3/B2/B3/B4 all of which are covered by A3/B4. If 10w40 is specified, use this, its fine, the engine was designed around it. You can use 5w40 too, probably 0w40 in a low mileage or rebuilt engine, some good examples below.

Now onto my playing field, D5 euro 3, and others calling originally for 0w30 A3/B4.
0w30 A3/B4 is rocking horse poop nowadays, I can only find 2 oils meeting that spec that are actually readily on sale in the UK! Even then its mail-order.
This is how a lot of folk are inadvertantly putting A5/B5 in when they shouldnt.
As such, I have compiled a list of readily available oils meeting A3/B4 spec and permitted viscosity in our UK climate. (Some of these oils, 0w40, 5w40 will also be ok in older engines calling for A2/B2 A3/B3/B4 as above.)
The first 3 are seemingly the more ideal oils as they combine with Api SL too.

0w30 (A3/B4/SL)
Total Quartz 9000 Energy
Fuchs Titan Supersyn Longlife

5w30 (A3/B4/SL)
Castrol Magnatec stop/start

0w40 (A3/B4/SN)
Castrol Edge
Fuchs Titan Supersyn Longlife
Mobil 1 FS
Millers XF Longlife

5w40 (A3/B4/SN)
Fuchs Titan Supersyn
Mobil 1 Super 3000 X1
Total Quartz 9000
Shell Helix Ultra
Motul 8100 Xcess
Millers XF Longlife
Texaco Havoline Ultra

This list is not exhaustive, but is what is freely and (relatively) immediately available on the open market, there are others, but ive avoided obscure/cheap/expensive/exists but cant find a vendor or listing, etc.