partly because the engine compartment friction is so great that minor viscosity different is negligible after all. Unless your engine already posses these problems:

1) Loosen valve seat - engine oil leak into combustion chamber from engine head, that mixed up with A/F result in bad burning
2) Loosen pistong ring - engine oil being channeled into combustion chamber from crankshaft, that mixed up with A/F result in bad burning
3) PCV breather blocked - excess engine oil being stored on rocker head will leak into intake vale, that mixed up with A/F result in bad burning
4) PCV breather valve loosen - unable to block engine oil from staying on rocker head, excess engine oil being suck by intake manifold, that mixed up with A/F result in bad burning.

If your car's performance affected by engine oil, you need to get your car fixed because your car may be suffering from 1 of the above. I've been driving for many years, many kinds of cheap/expansive/racing grade/taxi grade engine oil I've used. The significant of an engine oil existed when you're in these conditions:
a) During engine cold, whether the oil has been freezed. Whether its sticky enough to stick on the metal surface to prevent your engine part from hard friction.
b) When you're driving in the afternoon + high heat. If you go 9000rpm, is your engine oil capable to sustain such heat without boiling. The hotter it gets, the watery it goes.
So, what kind of temperature you're driving on? Too cold? Or too hot?

There's also myth in the market saying some highly precised molecule engine oil can provide extreme lubrication that smoothen engine movement, the reduced friction will result in restore power output and increase fuel efficiency. I have absolutely no proof to say that's wrong. But then, what was the working temperature again? -20w50? You want high heat resistance, its going to freeze earlier. You want low freeze molecule, it wouldn't stand high heat. Simple as that.

Here's a video regarding independent engine oil test:


Basically, if my opinion was the difference of engine output is totally negligible. If you read it correctly, the 1st test of Mobil 1 was the most weak engine oil, but I guess many of us also realize as the engine run hotter the performance started to increase. If the shop owner willing to do the whole test again with the reverse sequence of the engine oil, started with Nulon, then Purple follow by Motul, Castrol and finally end it with Mobil, I would take a safe bet the Mobil is going to generate 180kw instead of the Nulon.

I personal preference was this:

Too bad, I couldn't find this type of oil in Malaysia anymore. Reason for selecting this is because it's very affordable, and placebo effect pretty good. It used to be NASCAR official engine oil, so I felt the car more powerful after knowing that I'm sure it has nothing to do with the actual performance, it just I felt better You may still find some detail here

If the car already have problem, such as loosen piston ring, this is my preference:

Affordable price, very hard to burn. If your car is a white smoker, this is just the thing you looking for before you get to repair your engine.