I diffuse lavender. It helps me relax and I like the smell.
Eucalyptus oil is great for fighting a cough and clearing congestion. Rosemary is supposed to help your memory, I'm not sure if it is efficient. Lavender for relaxation. Peppermint for energy. Tea Tree oil for fighting infection. Lemon oil helps boost cleaning power. I used woody scents when I made beard balm for my daughters' partners, they're all bearded. I use peppermint, and lavender most frequently.
I use 5wt30 synthetic oil in my Miata. It is essential.
I just got a diffuser. So far, the citrus are my favorite, especially tangerine. I use essential oil to make sugar scrub also...most in demand is Citrus Mint.