Dear,
I think u r not correct regarding 50 meaning 50 degrees outside temperature, these are oil standards and you will have to look for their meaning from relevant standard description. 5w50 isn't recommended because it is used for older engines, suzuki recommendations are 10w40 for Swift and you should use that. If you want fully synthetic then go for 5w40 or 5w30 they will also be good for fuel economy but 5w50 isn't recommended at all even by manufacturer.
Regarding plugs suzuki recommendations is Ngk ifr6j11 but they are not available in Pakistan so you can use denso ik20 or autolite 3923.
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