A reader asks: Can I use 75W-140 gear oil in my 2007 Harley-Davidson Road Glide transmission? Some say that the heavier oil quiets down the transmission. Is there any problem with this?
Our answer: It is perfectly acceptable to use 75W-140 gear oil in Harley-Davidson transmissions. But, do consider that 75W-140 is much thicker than oils typically used in Harley transmissions. For instance, AMSOIL V-Twin Synthetic Transmission Oil is an SAE 110 gear oil. Let’s discuss the pros and cons of going with 75W-140.
The reason that some Harley owners use 75W-140 is that the transmission tends to run quieter. The thicker fluid can dampen noises and rattles.
The downside to using a thicker oil is that it will hold more heat. Hotter transmission operating temperatures would be the result.
Wear protection– Using a 75W-140 will not improve gear wear protection. AMSOIL V-Twin Synthetic Transmission Oil or AMSOIL V-Twin 20W-50 Synthetic Motorcycle Oil can provide maximum transmission life. Wear protection doesn’t necessarily improve by using a thicker oil.
Shifting performance– There is no shifting performance benefit to using a thicker fluid. A thicker transmission fluid may actually make gear selection slightly more difficult.
How Does 75W-140 Viscosity Compare To Other Transmission Oil Options?
Here are the viscosity numbers for AMSOIL V-Twin 20W-50, V-Twin Synthetic Transmission Fluid and Severe Gear Synthetic 75W-140. The oil viscosity is measured in centistokes (cST) at 100°C.
- V-Twin 20W-50- 18.5 cST
- V-Twin Transmission (SAE 110)- 20.7 cST
- Severe Gear 75W-140- 27.2 cST
Note that different scales are used to measure the viscosity of motor oils and gear oils. That’s why the 20W-50 motorcycle oil’s viscosity is so close to that of the the SAE 110 transmission fluid.
Also note that the 75W-140 is a whopping 47% thicker than the commonly used 20W-50 and 31% thicker than the (SAE 110) V-Twin Transmission Fluid.
In conclusion, 75W-140 can certainly be used in Harley-Davidson transmissions. We’re not sure that the quieter operation is worth the hotter operating temps and (arguably) stiffer shifting.