You could be remembering a time when McD's used to cook their fries in beef tallow. They switched to 'vegetable' oil around 1990 [1] due to the persistent myth that animal fats promote heart disease, or at a rate higher than other oils.

IMHO, tallow/lard fried fries taste way better, and McD's really messed up here.

[1] https://www.thedailymeal.com/1020277/the-mcdonalds-fries-sca...