"We cook meat with oil."
Translation:Nous cuisinons la viande avec de l'huile.
In this context "la viande" means all meat that we cook, since there has not been a meat specified and it's not just some of the meat that we cook that we cook with oil, it's all of it.
And it's "de l'huile" because this means "some oil". If it was "d'huile" it would mean "of oil" and if it was just "l'huile" I think it would mean "all oil" since there is no oil specified.
If there had been a choice of oils, I think you would be able to say "l'huile" to specify your choice of oil, but I'm not positive of this.