9. Everything had seemed about normal for a trans with 120,000 miles on it until Joey removed the input shaft and main cluster. He found that the gears were burned up (and Sixth gear was worn), and would need replacing. We asked Eric Skillman about this and he said it often happens due to heat caused by low or dirty fluid. Anahiem Gear recommends that manual transmissions get serviced every 15,000 to 20,000 miles. This may sound excessive, but fluid is cheap and transmissions … not so much.