As a chemist please allow me to add my 2 cents fellas:
In general 10w-30 is a much more stable oil than 5w-30
(assuming its otherwise a similar formulation)
Reason is less viscosity index improvers are used/needed to bridge the gap form the Xw cold run viscosity to the warm running 30 weight.
What IS a multiviscosity oil?
A 5w-30 is a 5w oil thats been improved so it only thins out as MUCH AS A #) WEIGHT WOULD when it gets warmer.
Thats whats called multivis
This is easier to accomplish over smaller spreads like 10w-30 has less VIIs
Why does this matter?
VIIs dont add anytthing to lubrication itself , so the less you have the mor eoil you have in a given quart formualtion.
This is good
Also VIIs decompose into varnish
This is bad
The thing about syntheic oils is that they need much less VIIs to make multivis bahviour happen.
This makes possible the excellent Mobil1 ow-40 and Valvoline Syn 5w-40 .
So you cna get more multivis bavhious w/o overloading the formulaiton w/ VIIs
Conversely a syn 5w-30 has very little VIIs and many synthetic 10w-30s have no VIIs at all.
I used ot race lot and use a fill of Mobil 1 0w-40 all year around, or redline 5w-40, which are both protect great against viscosity break down AND cold startup.
The VQ series of engines does not have the hotspots of some some other engines but even here I prefer to run 10w-30 year round in milder climates.
10w-40 maybe Valvoline Durablend in hot climates, or Castrol Syntec 0w-30 in cold climates.
(The VQ37 in the Z and G has hotspots and runs hot, thats why they only want to see a 5w-30 in it if its a very high end ester oil.. w/o high end chemistry you cant make a 5w-30 work well in those hot engines. The option to just go with a Xw-40 weight Nissan cant take because CAFE gets calculated based on factory fill, and this is whats responsible for the manual saying 5w-30 only. )
Just FYI in other countries Nissan still recommends changing the viscosity w/ the seasons like it used to do here.
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