hi all,
im in need of engine oil and i ussually run 15w40 magnatec castrol.
no issues but i see castrol also say i can use 10w40 magnatec. only reason im considering this is cause its a good deal at the moment through repco.

these are my two options i was wondering what i should do?
would 10w40 be better for the engine or fuel economy? or are these oils too similar to notice a difference.

thanks