Brown flax is the furniture oil - but it can also be 'eaten' just as the other which is known as yellow flax.
Linseed is the other name for flax but I always use both names when nagging people bout he benefits of some oils in case they only know of the one name for the product ( I regularly berate shoppers at farmers' markets I attend - lately its been about good for you oils, recently its that ASDA flog you a frozen cheesecake with 68 ingredients, and so on...)
Quite right to change oils after 6 burns or so. Some of the extended life oils only have extra life as they have added vitamin E, but sunflower has this as a natural component. Even so, best to only use for specific time.
You can of course improve it by filtering when cold after a fry - its what tights were invented for.....
If you fancy getting a bit of other info about healthy oils have a look at :
http://whfoods.org/foodstoc.php
All oils have a flash point some higher than others so are better for frying and they can also deteriorate on heating - and go rancid. There are a load of resources on the web that might advise or if you want a list of oils one starting point is
http://en.wikipedia.org/wiki/List_of_vegetable _oils and follow the links.
Hope this helps..