Please don't do it.
Continue with what grade you are using, i.e. 5w-30. At max., change brand, but not grade.
I have myself experimented it on my ex ride, and literally repented.
IMO, irrespective of tyre size, 14" size rim is best suited for Vitz, considering our road conditions.
The moment you switch to 15" rim size (1500 cc Vitz is an exception), and tyre size being 185/60/R15 or 185/65/R15, ride becomes hard, uncomfortable, noisy and bumpy, and prolonged exposure to such condition may fail different suspension parts prematurely.
Dunlop / Bridgestone tyres can aggravate the situation.