Bro because i have seen mecahnics do chilling and chewing gutka while asking chottu, "plug hore taaait kro", thus messing the threads, evetually u would be left with plugs at different heights at diff cylinders. Not a good situation.
On a dead cold engine, after tighteing with hand without an extension, u would just need 1/16 th of a complete turn to achieve desired results in case of a missing torque wrench.
But as per hazrat mechanic " jb tk insaan ki bs na ho jae, tait krtay rho". That is in plain words "mardaana taaqat azmana".
Anyhow getting to the actual point, if 5w30 gives no issue then use fully syhthetic 5w30.