single-grade oil

i live in the tropics (equator) where we seldom see temps go down below 15deg C (59deg F) on the coldest of days/nights here and so was wondering, instead of using the multi-grade 10w-30 or similar oils for my fte engine, can i just swap that for a single-grade oil like SAE-30 or -40 oil...

i live in the tropics (equator) where we seldom see temps go down below 15deg C (59deg F) on the coldest of days/nights here and so was wondering, instead of using the multi-grade 10w-30 or similar oils for my fte engine, can i just swap that for a single-grade oil like SAE-30 or -40 oil...