Technical 0W-20 C5 oil

[murphyv310]

Amazon sell Castrol Magnatec 5W-40 C3:

https://www.amazon.co.uk/dp/B07ZKJXK6R/

Any reason why I should source Selenia over this to top up my Panda 4x4?

In a word no.
Let's not forget when you do an oil change not every drop is removed from the sump and other pockets and parts of the engine. Purists use a flushing oil but that too cannot be fully drained. One of the many reasons I advocate halving the oil change frequency.