SC uses 5W20 engine oil for my car, will there be any effect to the car engine in long term use? Since its quite thin..

you're only addressing temperature, this is addressing fluid longevity and viscosity/friction modification.
additives have their place, in fact they are pre-added to almost all products you buy on the market.

they are just not commonly encouraged/prohibited by most oil manufacturers because they simply cannot stand behind/vouch other brands/products compatibility.

then there's the issue of cost.
For example: using amsoil signature series is way cheaper than say, motul 300v, however if i add in nanolube ws2 and liqui moly ceratec, it easily goes well over rm500 and thats easily way more expensive than motul 300v which already have very good additive package and a strong ester based base oil.

see, manufacturer specs the ATF to be serviced much longer than 20k km (even the old 1990's 3 speed autos are specced 40k km afaik), however with how conventional ATF is along with our weather/traffic jam, even 20k km may not be sufficient.
for example, my 1999 iswara 3 speed auto when using purely rm20ish/liter proton ATF(spIII), it only last until 5k ish km where i get some car "transmission morning sickness" where by it has a huge delay in shifting until the car warms up.
so upon draining, the fluid is very watery (just visual alone it looks like water as opposed to being oil  and it flows rather fast as well) and is brown/black in color as opposed to deep red and smells absolutely horrible.

5k km.
lubegard alone on the same ATF can extend this to easily over 20k km and even upon draining the fluid doesn't look watery and doesn't smell as bed, and still retains the red'ish color.

I think your 3-speeder is dying. I've never had this sorta problem with mine

naw, its just how one abuses the oil - if it would give you an idea, i treat the box as a manual box without a clutch pedal during my active days on the iswara. L/2/D, 1/2/3 3 speed, aye?

i've gotten it to overheat as well during sepang where it couldn't shift 3rd until the slush box cools down a bit/lower my rpm

the same 3 speeder box is still being use till this very day without a repair/overhaul by myself/my sister

I don't drive it the way you do, but I used to drag the gears for long periods of times. Like keeping in on "2" on the highway so I could get the instant kick anytime I needed it. I've cooked my gearbox a lot of times and it wasn't on track. Abuse wise perhaps we're comparable but for the oil to wear out after 5k, either you're using fake SPIII oil or you never change the ATF filter often?

What about the filter?

What about the filter?

I've never heard of a manual overheating on a normal car. On those high powered cars got la.

Hmm..so i discover this recently.

http://www.kixxaustralia.com.au/products/o...-clean-4-litre/

Thoughts anyone? if i am not wrong, basically this is a engine oil with concentrated cleaning additives. Drain old/used engine oil, and fill this as engine flush, let run for a period of time and drain. Only then you fill it in with the actual EO for daily usage.

MSDS pun ada, macam normal "EO" only.

Aeon big has promo

Mobil 3000. 5 litres. 5w-40.fully synthetic
Prior 18 to 31st Dec 2015

Aeon big has promo

Mobil 3000. 5 litres. 5w-40.fully synthetic
Prior 18 to 31st Dec 2015

Rm149