Your number is exactly correct (1.4kg/day) [1] but you can easily do it without oil as they did in the study (there are a lot of waking hours in the day and a lot of good fatty foods.) Alcohol (and some medication) of course makes both storage and consumption easier.

1. https://pubmed.ncbi.nlm.nih.gov/7598063/

3 lites of oil is approximately 10 times the recommended calorific input for adult men; it certainly isn't "easy", on the basis of what I've heard from a now-long-deceased acquaintance with cystic fibrosis who was required to consume 10,000 kcal/day just to overcome the medical condition; and 10,000 kcal/day is less than half the amount here (24,436 kcal), which I chose on the basis of it being a lot of fluid to drink in one day even if it was water.