I was going to suggest olive oil. Instead of in the food, put it in an eyedropper and squirt it down the cat's throat. She should also plant some kitty grass. It comes up in a few days, and is great for their digestion and their hearts.

But with a kitty that age, kidney issues should always be considered, especially with a male. If feeding dry food, stop it right now. It can cause urinary tract crystals that can block the pee, and they will throw up.