Does the time difference between Mumbai and São Paulo change during the year?

No. The gap stays at 8h 30m all year.

When is the best time to call São Paulo from Mumbai?

There is no overlap between standard working days, so one side has to call early or late.

Mumbai is 8h 30m ahead of São Paulo. This gap is the same all year — neither city shifts its clocks relative to the other.

Mumbai

India

--:--:--

São Paulo

Brazil

--:--:--

A 9-to-5 working day in Mumbai does not overlap one in São Paulo at all — somebody has to take the call outside office hours.

City	Time zone	UTC offset
Mumbai · India	Asia/Kolkata	UTC+5:30
São Paulo · Brazil	America/Sao_Paulo	UTC−3

What is the time difference between Mumbai and São Paulo?: Mumbai is 8h 30m ahead of São Paulo. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between Mumbai and São Paulo change during the year?: No. The gap stays at 8h 30m all year.
When is the best time to call São Paulo from Mumbai?: There is no overlap between standard working days, so one side has to call early or late.