A co-worker of mine had a rental for over a month, requiring two oil changes. He called and asked, they sent him to an authorized oil change place and had it billed directly to Avis. All he had to do was drive it there and get it done.
Either way, you should probably just call the Avis location you'll be renting from and see what they want you to do. They may give you a car that's just been changed, and most cars don't need one under 7,500 or 10,000 miles now.