- 50 at operating temp doesn't mean high fluidity, it is an oil suitable for forced induction or high compression engines that see high egts.
40 weight oils are recommended for even the old cultus so 50 is overkill for vitz
any oil that has low operating fluidity also has lower fluidity at cold compared to lower viscosity oils i.e. 5w30 or 5w40 oil
the higher the difference between two numbers the more early an oil deteriorates as it is difficult to engineer an oil that gives best of both worlds so one won't be able to get as high drain intervals as a synthetic oil should provide.
of course higher viscosity oils cover noises made by an engine that's why dealers pour them when trying to sell weak engine cars.
last I don't have experience with this oil in vitz so if it works for you it should work for the OP
but my experience with it in Altis grande 1.8 (the car for which it was original launched by Toyota Indus) wasn't good and opted for ZIC X7(A+) after 3rd oil change and noticed improvement in starting(twice, the car need second self to start few thousand kilometers when cold and ecu registered a code cleared at dealership which never happened after switching to Zic A+), improved performance and a huge improvement in fuel economy. last the condition of the tagalon wasn't as good at 4000km change interval as I've found of Zic A.
for OPs ride, he should opt for any good brand 5w30 in my opinion or petron plus as the 2nd best option.