SPECTRO Heavy Duty Platinum 6 Speed Transmission Lubricant GL-5Full Synthetic Transmission Oil, For Helical and Straight Cut Gears. In response to Harley Davidson owners complaining about their great, but noisy, new 6 speed transmissions, Spectro developed this Heavy Duty Platinum 6 Speed Transmission Oil. Fully synthetic, it is formulated to significantly reduce gear noise and provide easier, smoother shifting even when cold. High loads and extreme heat do not affect the oil, so it performs well when pushed hard