I don't know if it's just me or what, but I usually drive long distance, say 6-8 hours regularly, in hot M'sian weather, as we all know it.
From my observation, if I'm using 30 weight oil, doesn't matter if it's mineral 10W-30 or semi/full synthetic 5W-30, as per recommended in the manual, during those long hours of driving, I can hear the engine revving 'rough', like the engine was suffering screaming arrrghhhh or something like that, hahah.
I do understand about the recommendation of oil weight in relation to the engine's clearance, but out of geramness about the rough revving sound, I changed to Shell Helix Ultra 5W-40, and walla, even after hours of driving under the blazing sun, the engine feels & sounds smooth, no straining screaming rough revving, the engine was like: "chill brah, I'm good".
And on one trip with my friend who's driving the new 2019 Myvi, that was using the centre's 0W-20, man.., the engine sounded so rough for a new car, screaming like it was dying..
That was my experience btw, so in terms of engine's clearance vs ambient temperature, I think the SAE engine oil weight recommendation in relation to ambient temperature does holding some truth.
This is good information from the point of user. Cause all these cars when they test in the lab usually in controlled environment. However, for cars like Perodua I am sure they would have tested them under Malaysia's climate. It is strange for the recommended oil to be not suitable for Malaysia environment. Anyway, it is best to stick to manufacturer's recommended oil rating. Change it to higher only if you know what you are doing like in this case.