A transmission rebuilt by a competent mechanic should be as clean and serviceable as a new unit. Check the service interval recommendation for your vehicle, and follow it. Depending on the make and model of the vehicle, the recommended service interval may range from 25,000 to 100,000 miles; some cars may never require this service.
Transmission rebuilds can become expensive; replacing your transmission will cost even more. For peace of mind, if for no other reason, change the transmission oil and filter in a rebuilt transmission after 5,000 miles. Some shops require a 5K oil and filter change as a condition of their warranty and will include this work at no additional charge.
Change a rebuilt transmission's oil and filter at 5,000 miles---especially if not doing so will void the warranty. After that point, rely on the manufacturer's recommendation for maintenance intervals.
