Mix 5w-40 and 10w-40 engine oil??

[kaz]

can we top up 10w-40 castrol magnatec with 5w-40 oil

I have got a 5w-40 Chevron supreme fully synthetic from Costco, which is £12, seems like a good deal to me, anyone knows if this oil is good at all??

Spec on the tin says specially designed for VW 505 specs and BMW etc

if its good I would stay with it as its 5w-40 and cheap, if its not then i need to think of getting someother brand as I am not happy with Castrol, used Halfords 10w-40 once and that seemed a lot better as castrol has increased the engine noise.....(mad?? is it just me going mad??)

gsf does total 5w-40oil but its 25+vat, i got castrol 10w-40 from costco for £16
chevron 5w-40 is 12quid