[ainarssems]

First digit is viscosity at low temperatures, the lower number means lesser viscosity so it gets lifted and provides oil pressure sooner. This is important at cold climates, you don't really need first digit to be 0 unless you are using car at temperatures lower then -30C. At UK temps(except Scotland maybe) even 15 for the first digit will be fine.

Quantum Longlife III 5W30 (or Castrol Edge equivalent) does good job for everyday driving and can be sourced relatively cheap. It is suitable for PD diesel engines which experience high pressures at camshafts operating PD units, they have plenty of high pressure and anti-seize additives so even at cold starts with no oil pressure there is no metal to metal contact so it does not matter if oil pressure is reached in 1 or 1.5 seconds

If you want better protection when pushed hard like on track you should be paying more attention to second number after W. You want it to be as high as possible to retain viscosity at high temps, 10W60 or 10W50 would be good choice in that case.