Hay guys. I just wanted to ask about oil grades. So if a oil is a 10w-50w. What weight of the oil it for when the cars hot and what weight it used for when the cars cold.? Cheers Kurt.

It means it flows like 10 when cold but flows like 50 when hot.....in other words it maintains it's stability and doesn't thin out like single grade oils which are no longer available.

"t means it flows like 10 when cold but flows like 50 when hot.....in other words it maintains it's stability and doesn't thin out like single grade oils which are no longer available. "

Not quite like that - look up the oil company websites and you will see that say a 10W40 oil has viscosity certainly thinner than a standard 30 grade oil at cold temperatures, but probably about the same when the oil is hot. I don't have the figures available now, but for what it is worth, 15W50 oil when hot (100 Deg C) has about the same viscosity as water, which is pretty scary. Gear oils are graded differently - 90 grade has a similat viscosity to 30 engine oils at 40 Deg C.

eHow explain it like this:
[I]Modern oils may have a designation of a number, letter and number, such as 10W40. This example is a multi-weight (viscosity) oil. That is, it will perform as thinner (10 weight) oil when your engine is cold and as a thicker (40 weight) oil when your engine warms, thereby protecting your engine when cold and warm.

Yes - the 10W part of the oil is thinner than the standard 30 grade oils when it is cold, but it is strill thicker than when the 10W oil is hot. Check out the oil company website and they quote viscosity at 40 Deg C and 100 Deg C. All of the oils are thinner when hot.

Didn't I say that in the first place?

A good place to go for more precise info is penriteoil.com.au then select Training - Fact Sheets - Oil Viscosities.
Note that the second number of a multigrade always relates to its viscosity at 100 degrees, whereas for the first number tests are at progessively lower temps. For example a 10W oil meets the test at -25 degrees while a 15W oil meets the test at -20 degrees.

There are graphs available to check the viscosity of oils at more relevent temps - eg widman.biz/English/Calculators/Graph.html.
Insert the viscosity at 100 & 40 degrees of one or more oils (as supplied by all companys except Valvoline) & it will estimate the viscosity in graph form & in 5 degree increments.