Using good engine oil is very important in maintaining the engine. If just need oil only then why don't you use cooking oil for your car? Cheaper.

Without going into details on what comprised of good and bad engine oil, you just need to know good oil have addictives that help make the oil work better or last longer. Once you put the oil in the engine it will get to work very hard in protecting your car engine. It is important to have good quality oil as bad oil will cease to function as lubricant and also other effects. Bad oil will cause engine wear and tear faster which cause the engine oil to leak as mentioned.

As a few expert said before, if you have the money change the oil every 100km. It is always good to have fresh oil. For cost reason usually people will just use the oil in it's recommended intervals which is semi-7.5k and fully-10k plus or minus few k. Some oil can go up to 15k but it's up to you to experiment.

For an old car like your myvi, it is true you should use something with higher viscosity. Recommended in manual is xW30 but you can use xW40 also. I stick to using 5W30 for my 2006 Myvi and it is working fine. However, I am testing out a few cheap local made oils. I would recommend using fully synthetic as it has better quality oil and additives. Last longer too so I don't need to change too often.

I am not going for known brands for fear of fakes. Lots of fake for perodua, shell, petronas, honda, toyota brand of oils. So go for brands that nobody bother to make fake.

If you search this forum, you can find people discuss about oil blotting tests. Over there you can find a few reviews from real users on oil quality.

What's a roker valve gasket?

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.