Increasing fat?

[Linds]

I am experimenting with some new diets for my rats and mice. The diet I am currently trying out seems great, the ingredients and fiber/protein levels seem suitable, but the problem is the fat level is too low (2.5). What are some efficient ways of increasing fat levels in their diet? I was thinking of perhaps lightly coating the chunks in oil prior to feeding, but wasn't sure if that would cause them to spoil if left for two or three days