About the A5/B5 oil: These fuel economy oils have a reduced shear resistance, therefor increased engine wear. Should the manual allow other types of oils I would use a good A3/B4 oil.
A good indication for a premium engine oil would be the Mercedes "MB 229.50" or the Volkswagen "VW 504/507" specification. Also the "API-SN" mark should be present. The brand (shell/castrol/whatever..) does not decides about the performance, just the price.
You can compare the specifications by this tool: https://online.lubrizol.com/relperftool/pc.html
Just for clarification:
In my book the A5 specification was invented to lower the fuel consumption by lowering the HTHS viscosity. While an engine that demands A5 oil is supposed to handle the less wear protection (by better design) a good oil with high HTHS (if the manual allows it) increases the engine life.
It is difficult for me: I personally would put in something different than an A5 oil, but it is against the manual and I do not want to suggest it to you. Perhaps you should just stick with the recommendation from the manual and use an A5 oil. Skip the "Brand X"-part although, that is the least important thing.
Just for reference:
- A3B4. HTHS Viscosity>3.5 mPa×s
- A5B5. HTHS Viscosity>2.9 mPa×s
HTHS means "High Temperature High Shear" and is one of the more important factors on engine wear.