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Basically, engine oil is blended with base oil and additives.
The rule of thumb is that the higher the difference between low temp. index and high temp. index, the more additives you need to cover the change in viscosity. For example, comparing Mobil 1 SAE 15W50 and 5W50, you can get lower viscosity at room temperature with 5W50, with the expense that the lubricant is actually requiring more VII (viscosity index improver) to set. The VII is then the key ingredient to keep the lubricant's viscosity, and hence the oil film stability. VII is one of the easiest members to be decomposed by heat, and shearing force in the engine..... So, you'll have the lubricant get earlier deterioration....
So, good lubricant requires GOOD BASE OIL, to minimize the dependence on additives.
And don't be fooled by the API SM standard, NOT all lubricants are made the same although they can fulfill API SM standard.
The aim of API SM is actually to REDUCE POLLUTION..... It is not talking about increasing performance of or protection to engine. Real demanding lubricants can never achieve API SM requirements as at least one of the additives required for engine protection, such as zinc compound, can never be reduced.
So, watch out and choose.....

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基本上每一隻機油都有潤滑作用, 新的時候一定好好.
不過, 分別在於其可抵抗褪化的能力和承受嚴峻的引擎環境.
先講性能高的機油
1. 它可以承受凸輪軸和曲軸高速旋轉時產生的離心力, 保持依付而仍有潤滑作用.
2. 它可以承受高溫而不致breakdown
3. 它的油膜可以很薄而仍然有潤滑能力, 令滾動阻力減低, 可以進一步慳油.
4. 它有抗酸能力, 防止電油燃燒後產生的Nitrous Oxide侵食引擎內的金屬部份.
5. 它的褪化周期很慢, 不會頭三四千公里便開始立.
6. 它可以防止起泡, 可減少阻力.
7. 它可以有足夠依付力不會在熱車變凍車時流晒落油底, 在凍車一撻時作出保護.
8. 它不會蒸發得太快.

我記得的大約是咁多.
沒有添加劑和油底較酋的一般油, 在2000-3000轉的秋天駕駛, 應該仍可以使用3000-4000KM, 經常換油是可以保持引擎效能.
逢星期日...糕-糕-糕.

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Dear Simon, Elvin, and all the other experts, any comments?  My friend told me he got extra mileage from his car (foot car)after this.

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