It is often misunderstood but 0w40, 5w40 or 10w40 all have a very similar viscosity whilst the engine is at operating temperature (the 40 part).
All oils thicken when cold, so you will get less thickening with a 0w40 than a 10w40 oil on start up, for example (the bottom number).
Oil technology has moved on and I would suggest that you key in your registration number into a leading oil manufacturer's website such as
https://lubes.mobil.com/uk-english-l...which-oil.aspx
With a properly maintained engine, I would avoid using a "flushing oil" as this has a viscosity close to tap water.
It would need sacrificial oil changes to get rid of the contamination from it.
These products are only necessary as a last resort for a neglected, poorly maintained engine, which have not had recommended oil changes.
This particularly true on time intervals with low usage.