immediately. The transmission fluid is highly filtered and in fact, the contaminant level declines throughout the service cycle. In addition to extending the service cycle... is every 12 months. It is also advisable to renew your transmission fluid at this time. In fleet applications a test by the Ford motor company demonstrated fluid and filter could... be 30,000 miles. (Each transmission runs differently and may require different fluid service intervals.) At change interval you simply evacuate (or drain) the fluid from the system...