Carene,fish oil,i prefer salmon oil, has one of the highest calories as far as oils go,if you want to put weight on your dog, fish oil is a great all round oil for dogs IMO.
The thing i dont understand is "how much" oil to feed to actually be able to put weight on a dog,i would think it would have to be alot everyday to see any weight gain

My guys get 2-3 capsules of salmon oil daily,which equates to around 1tbs.,it hasnt made one iota difference to their weight.
I would really like to hear from someone who has achieved weight gain in a skinny dog just by feeding extra oil :)
Here is a list of oils/calories:
902.1 calories per 100g
beef tallow, lard, mutton fat, cod liver oil, herring
oil, menhaden oil, salmon oil, sardine oil
900.1 calories per 100g
chicken fat, duck fat, turkey fat, goose fat
hope it helps.
Edie
XXX