The M8 transmission case will hold approximately 32 ounces, or 1 quart of Harley transmission lubricant. However, unless the transmission was completely disassembled, there will still be an undetermined amount of residual Harley transmission lubricant remaining in the case.
That is why; to refill transmission case, it is recommended that we start with 28 ounces of Harley transmission lubricant. Park the motorcycle on a level surface on the kickstand. Add 28 ounces of oil, then, thread in the dipstick/fill plug until the oring contacts the case but do not tighten.
Remove the fill plug and check the lubricant level on the dipstick. The proper oil level is achieved when the oil level is in between the ADD (A) a Full (F) marks. Add only enough fluid to bring the level to between the A and F marks. Then, install the fill plug and tighten to the factory torque specs of 25 to 75 inch pounds.
We believe that Spectro’s Heavy Duty, Platinum, 6 speed transmission lubricant is a god substitute for Harley Davidson’s Formula + or the Screamin’ Eagle SYN3 full synthetic oil. Spectro’s 6 speed Harley transmission lubricant is a GL-5, 75w 140 weight fluid. It was designed by Spectro in response to complaints from Harley Davidson owners about the noisy 6 speed transmission. The oil is full synthetic. It significantly reduces gear noise and provides easier, smoother shifting.
Harley suggests that the Harley transmission lubricant be changed at twenty thousand mile intervals after the initial one thousand mile service. We, however, suggest that the transmission oil is changed every five thousand miles or once a year, depending on which event occurs first. If the motorcycle is ridden and/or stored in a damp environment, we would suggest changing the transmission fluid even more frequently. Remember oil is relatively inexpensive as compared to a repair or rebuild.