Great questions...
i)Does each engine require specific engine oil viscosity grade?
The oil grade is determined by the minimum oil film thickness necessary. The minimum oil film thickness is determined by 3 main parameters...
- Operating oil temp
- Rod and main bearing clearance
- The load on those bearings
...with other parameters like oil ring tension and hydraulic lifter bleed rate being secondary parameters.
ii)Does each engine has specific engine oil pump design? How does oil pump design determine oil viscosity grade?
There are several oil pump designs from different manufacturers on different engines, but they are all alike in that they are all positive displacement pumps. The pump has little to no impact on the oil grade.
iii)If I use different oil viscosity grade than the one mentioned on the car manual, any effect to the engine performance/life?
That depends on how drastic of a change we are referring to. If the manual calls for a 5W-30, and you use a 5W-40, it's unlikely you'll have any issues. However, if you use a 15W-40 while trying to cold start it in -30°C weather, you're probably going to have a bad time. In some cases, you can even go to a lower oil grade and be okay. I recommend you exercise caution in this regard though. In general, you're better to lean to the excessive side with viscosity rather than the insufficient side.
The recommended oil grade in the manual can be full of compromises like CAFE appeasement and CYA efforts. Sometimes finding the oil grade that best suits your driving style and use of the engine is needed.
iv)For a passenger car don't have VVTI system and operate at 10-to-40-degree Celsius environment condition, any effect if use 0w30/5w30 viscosity grade engine oil?
Abstractly, the only difference between a 0W-30, 5W-30, and 10W-30 is the dynamic (pumping) viscosity in cold weather. The first number represents the lowest temperature at which that oil remains pumpable, hence the "W" stands for winter.
0W-xx = -35°C
5W-xx = -30°C
10W-xx = -25°C
15W-xx = -20°C
20W-xx = -15°C
You can use a 0W-xx or 5W-xx in a climate (and application) that calls for a 10W-xx. However, you cannot run a 10W-xx in a climate that calls for a 5W-xx or 0W-xx.
In a practical sense, a 0W-30 and 5W-30 oil will generally have a lower viscosity initial base oil with more viscosity modifier (in the form of polymers) added to achieve the wider spread. This can make the oil less shear stable and increase the volatility which can lead to more carbon deposits and oil consumption. I tend to only recommend the lower winter grade when it's necessary, and stick with the narrowed spread I can.
v)For old truck (run 100k miles), use of more advanced oil(synthetic) could help to extend oil drain interval?
A synthetic oil will always have a longer service life than conventional counterparts.
vi)Any effect of use engine oil treatment (STP) to engine performance/life?
Only negative effects, for most cases. STP Oil Treatment is a viscous polymer with only a trace of additives. It will increase the viscosity, but will also dilute the additives already in your oil including your anti-wear, anti-friction, detergent, anti-oxidant, and anti-foaming additives. I generally recommend against supplements of any kind. My theory is if you need to add something to your oil, then you need to use a better oil.
vii)If car/truck don’t have manual for oil specifications, how to know which oil to use?
In such cases, I tend to default to a 0W-40. It's a middle of the road grade that is unlikely to cause any harm in any engine.