I found that a gradual transition has been working well for me, since I had trouble eating and digesting fats. I found tallow, bone marrow and raw eggs to be easier to digest than raw suet, intramuscular fat, or fatty sausage. Coconut oil made me ill, but some do well on it. I also had to take potassium and zinc supplements to correct the deficiencies from past diet and damage done by a past prescription medicine until my GI was sufficiently healed and I was eating substantial amounts of raw meat and tallow and had cut out the carbs completely. I guess tallow has been the key to the transition for me and reaching mostly raw zero carb has been quite a health bonanza so far.
Cod liver or fish oil makes sense if you're eating animal fats from grain-fed animals that contain lower levels of omega 3s. Too much cod liver oil can build up excess vitamin A levels, so just don't go overboard if you take that. I know someone who uses it regularly and says it helps with his arthritis. Of course, if he ate a raw meats/fats/organs diet he wouldn't have the arthritis.