Oil Viscosity...Please explain 5W-50, 10W-30, what does each number mean?

[Berlina 5.7]

Hi all, i have tried to do a search on oil viscosity but can't seem to get the info i want. If you take an oil like 5W-50, does this mean that at cold (5W) the oil is thiner, but at operating temperature the oil gets thicker (50).
Each person i spoke to gives me a different answer. I was also told that it is impossible for a thin oil to get thicker when it is hot. I can't seem to find the answer i my previous searchs. Can anyone help.

Thanks

[Rt!]

Its quite simple
the oil is behaves like a less viscous liquid (5) when cold
but behaves like a higher viscousity liquid (30) when warm
this doesn't mean that when the oil is warm, it is thicker than when it is cold.
Think about how most standards are set.. its at a constant temperature (25deg C generally IIRC)

[Mongy]

If you go to this website
http://autorepair.about.com/od/motoroils/
It supplies quite a lot of info about oils, with handy links

[SSbaby]

10W-30

Just think of the 'W' as 'winter' (as in the season). Generally, for colder climates, the thinner oils (lower 'W' rating) are more suitable for max engine protection. It's the other way around for warmer climates.

Not that long ago, both Ford and Holden recommended owners use a 20W-50 oil. It wasn't until the introduction of the AU that Ford claimed better fuel economy because of the switch to 10W-30 oil from the factory. Holden were less than impressed with Ford's claims and decided to follow suit.

[Speedy Gonzales]

The lower the 1st number, the quicker an oil pumps up when starting, the higher the 2nd number, the thicker an oil is at operating temperature.

Though this is not always the case when looking at the specs of an oil. When selecting an oil, take into account:

1. Season - Winter, Summer etc

2. Driving habit - Cruising around, flatchanging and redlining

The following should explain.

http://www.infineum.com/information/tables.html

http://www.infineum.com/information/guide.html

http://www.infineum.com/information/terms.html

http://www.infineum.com/information/acronyms.html

[MountainRunner]

You also have to cosider the engine power when selecting oil. The more power you have the higher you want the second number because of bearing loads. Thats why the Castrol Formula R 10W-60 is rated for extreme engines. The 10 gets it around the engine fast, the 60 protects it under high loads.

That fuel economy stuff is good marketing for Joe public, but us high performance modifiers have a different set of requirements.

[spank]

i spoke to a mobil rep about mobil 1 , he says that the main reason for hsv to use it was to get fuel consuption figures down on the windscreen label that they have to have now . thicker oil causes fuel consumtion to up
depends what you are using the car for highway cruising or racetrack

[xshore]

To that guy that told you oils can't get thicker when there temperature increase, its true but then how do they control viscosity as it gets so hot ?

ive been told one way is they use polymers which maybe slightly out of phase as the bulk, when the temperature increases the polymer becomes in phase and adjusts the viscosity.

[Falchoon]

If your handbook reccomends say 5W-50 oil (normal mineral) and you want to run synthetic do you use the same grade?

[MountainRunner]

As a guess I would say yes. I went from a mineral oil to a Synthetic with lighter figures and it blew clowds of smoke. I think on a worn engine you need to use the same or thicker.