engine oil

i know its been asked and on thread before but what engine oil. mines a 1.8 has been used to running on 10 40 carlube brand from my motor factors but in asda there selling 4l castrol for 13 pound theres 5 40 way to thin 10 40 and a 15 40 would my car benefit from a 15 40 thicker oil its just under 100k and alls running smooth just someone said older higher milage cars runner better on thicker oil? or should i stay with the 10 40 also is anyone else using castrol in yours?

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