I think you will find "Nutrients" convers anything needed for a living organism to grow. This includes Fats, Proteins, Carbohydrates, Vitamins and minerals.
So yes i stand by my statement that mutton bird oil contains nutrients and is much more than a calorie booster.
i think you will find vitamen A, D, and E can all be tolerated in high doses quite readily and are only "higly toxic" in extreme situations under extenuating circumstances such as other disease or ailment. Vitamin D is of most concern as it could be consider a prohormone and not a vitamin.
Vitamin A: "While in general the body absorbs retinoids and vitamin A very efficiently, it lacks the mechanisms to destroy excessive loads. Thus, the chances for toxicity exist unless intake is regulated carefully. Revision of earlier estimates of daily human requirements of vitamin A has been suggested; the suggestion is that estimates ought to be revised downwards. Concern exists about the teratogenicity of vitamin A." (humans only accounted for in this paragraph)
Vitamin D: "Vitamin D toxicity can cause nausea, vomiting, poor appetite, constipation, weakness, and weight loss. It can also raise blood levels of calcium, causing mental status changes such as confusion. High blood levels of calcium also can cause heart rhythm abnormalities. Calcinosis, the deposition of calcium and phosphate in the body's soft tissues such as the kidney, can also be caused by vitamin D toxicity."
Recommendations for Vitamin D are EXTREMELY low - your snakes should be able to get adequate Vit D from being in the sun for 15 minutes per week!
Vitamin E: "While short-term supplementation with vitamin E up to 1600 IU appears to be well tolerated with minimal side effects in most healthy adults, the long-term safety is questionable. Recent data suggest a possible increase in mortality and incidence of heart failure with long-term use of vitamin E (400 IU or more) in patients with chronic diseases."
I did copy and past all of these fromwebsites to save me re-typing what i was originaly taught in Toxicology classes at uni.
It's not on the topic of mutton bird oil - but I would be inclined to say I haven't an informed choice on this matter, but I am fully aware of nutrient toxicity
sorry I couldn't be of further assistance.