Since it was my post you are inquiring about, I will give it a shot. 
Answers inline below:

Ashley Moran wrote:
> On Apr 06, 2006, at 8:58 pm, Adam Sroka wrote:
>
>> Cooking walnut oil at high temperature will damage the oil and 
>> produce an unpleasant taste. Also, be careful with store bought 
>> walnut oil. Like other commercial oils it may have been treated to 
>> extend its shelf life. A sure sign that walnut oil is treated is if 
>> it comes in a clear or lightly tinted bottle. Walnut oil should be 
>> stored in a dark container in a cool, dry place, because the PUFA 
>> (polyunsaturated fatty acids) it contains are easily damaged by light 
>> and heat. Better choices for cooking include olive and macadamia nut 
>> oils since both are high in healthy monounsaturated fats and tolerate 
>> normal cooking temperatures. I like to use a blend of coconut and mac 
>> or olive oil, and occasionally palm kernel oil. I will leave out the 
>> coconut oil if I think that the flavor will offend, although for my 
>> own tastes I can't get too much coconut.
>
>
> Haven't bee paying much attention to the list lately - too busy eating 
> :) - so I missed this.
>
> I buy walnut oil from Tesco.  It comes in clear bottles and stays 
> fresh for ages.  I never heat it though, only use it for making 
> mayonaisse.  Is this healthy or not?  I *could* use macadamia oil, but 
> it costs twice as much and I'd be looking at £5 to make a small jar!  
> (Not that I'd mind paying if the alternative is own-brand 69p 
> vegetable-oil based glop.)
>

Mac oil is expensive. No question. It is cheapest in places like Hawaii 
and California where it is produced (Although the plant is only 
indigenous to Australia and certain parts of Indonesia.) On the other 
hand, it is very good for you. So, it's a classic trade off that 
everyone on this list has seen before in some form. Another note about 
mac oil: don't feed it to your dog; it is toxic to dogs.

The cheaper alternative to mac oil with mostly the same health benefits 
is olive oil. Extra virgin olive oil has a taste that can be obtrusive 
in many recipes. Lightly refined olive oil is a good alternative and 
even though it is refined it is still superior to any form of vegetable 
oil. You might also try palm kernel oil which is not terribly expensive, 
has a mild taste, and is fairly healthy (It has neither the benefits of 
mac and olive oils nor the dangers of vegetable oils.)

> On the subject of heating oils, I almost never eat nuts raw.  They 
> taste MUCH nicer roasted (the exception being macadamias which I think 
> taste about the same either way).  I take this to mean they are better 
> for me cooked than raw.  How does this stand with the idea of not 
> heating oils?  Whats the difference between heating the oil IN a nut 
> and heating the oil OUT of a nut?  If any?
>

I have heard, and am inclined to believe, that cooking nuts does not 
damage the oils in the same way that cooking the oil once it is 
separated from the nut does. Something in the nut protects the oil from 
harm. OTOH, this may just be someone's hypothesis that I happen to like. 
I am not aware of any studies that establish this. I am aware of several 
studies that show the oxidative damage done to PUFA when it is heated 
and the carcinogenic effects of the byproducts. Generally safflower or 
canola oils are used in these studies because they are both high in PUFA 
and cheap.

Hope that helps.