That depends on how long you plan to keep the car- if you are going to only keep it for 4-6 years or 100K miles, by all means do it once per year. On the other hand if you plan to keep it for the long haul, say 200,250,300K miles, i would change it more frequently, say every 6 months or 6-8 thousand miles, so the motor sill operates like new at 250,000+ miles.