A reader asks: A shop did some work on my Harley-Davidson and filled my primary with synthetic oil. I have always used regular H-D oil. Can I drain it and replace with regular oil? Will this cause any problems?
Our answer: You can safely drain the synthetic oil and go back to conventional oil (but read all the way down before you do). There will not be any problems whatsoever. Synthetic and conventional oil can be substituted back and forth without any compatibly issues or mechanical risks. This holds true for engine, primary chain case or transmission oils.
However, we suggest that you give the synthetic oil a try before draining it. You will likely notice that it is easier to find neutral. It is also likely that you will experience smoother clutch performance and cooler primary temperatures.
We are huge advocates of synthetics oils in Harley-Davidson motorcycles. Our customer feedback tells us about performance boosts in every imaginable category. We offer two synthetic options for Harley-Davidson primary chain cases. They are AMSOIL V-Twin Synthetic Primary Fluid or AMSOIL V-Twin Synthetic 20W-50 Motorcycle Oil. There is no wrong answer between the two products. Each offers identical performance and wear protection. Some like the idea of a dedicated primary fluid, while others like the idea of keeping just the 20W-50 on hand for engine, primary and transmission.
In conclusion, give the synthetic fluid a chance and determine if you experience any improvements. You can go back to conventional oil at any time in the future.
A reader asks: I’m doing the three main fluid changes in my 2005 Harley Fatboy. For the primary, I have used AMSOIL Synthetic ATF in the past. Is that a good choice?
Our answer: The use of automatic transmission fluid in Harley-Davidson primary chaincases has been a hotly debated issue for many years. Automotive ATF would not be our choice.
Why Do Some Use ATF In H-D Primary Chaincases?
This practice seems to have originated in the aftermarket clutch market and drag racing community. One of the main reasons is that the thinner ATF will allow the clutch to grab harder during aggressive riding.
Some aftermarket clutch-makers suggest ATF meeting the Dexron III spec, while others suggest Ford “Type F”.
We spoke to one aftermarket clutch-maker who suggested that Harley-Davidson Primary Fluid did not perform well with their Kevlar clutch plates. They claim that ATF is more compatible with their clutch plates (more on Kevlar clutch plates below).
We submit that ATF is too thin for Harley primary use. It’s true that many Harley riders have had years of problem-free performance with ATF. We contend that a thicker oil will provide superior metal-to-metal wear protection. Chains and sprockets will last longer using a fluid with an appropriate viscosity.
Below are the kinematic viscosity numbers for AMSOIL Signature Series Multi-Vehicle Synthetic ATF, AMSOIL V-Twin Synthetic 20W-50 and AMSOIL V-Twin Synthetic Primary Fluid. The viscosity is measured at 100°C.
ATF- 7.5 cST
20W-50- 18.5 cST
Primary Fluid- 18.7 cST
Compared to 20W-50 motorcycle oil and the dedicated primary fluid, ATF is very thin. For reference purposes, ATF has a similar viscosity to 5W-20 motor oil.
The AMSOIL V-Twin Primary Fluid is an SAE 90. Some may wonder how the 20W-50 and 90-weight primary fluid have a similar viscosity. The reason is that gear oils and motor oils are given different viscosity numbers on the label to prevent confusion. But in terms of the actual viscosity measured in the lab, they are very close.
In conclusion, if you have a stock clutch, use a dedicated primary fluid or a suitable 20W-50. If you have an aftermarket clutch, we would still submit that a good synthetic primary fluid is the better choice.
A reader asks:“I’m looking to switch my Harley-Davidson Street Glide Special over to AMSOIL. Obviously the engine will take your synthetic 20W-50, but I’m curious about what to use in the primary and transmission. There are a lot of different combinations put out there on the internet. Some use gear lube in the transmission and chaincase. Some even use ATF in chaincase and claim they have great results. What is the best combination for my bike?”
Answer: For any Harley Twin-Cam engine, we recommend using AMSOIL Synthetic 20W-50 in all three components for two essential reasons.
Update: AMSOIL now offers dedicated fluid for Harley Davidson primary chain-cases and transmissions. These are called AMSOIL V-Twin Primary Oil and AMSOIL V-Twin Transmission Oil. Should one use these dedicated fluids? Or is AMSOIL V-Twin 20W-50 acceptable for primary and transmission use? There truly is no wrong answer. AMSOIL V-Twin 20W-50 is designed for engine, primary and transmission use. This 20W-50 is certainly a highly competent and was designed to excel in all three components. For those that prefer a specialized fluid, the V-Twin Primary and V-Twin Transmission Fluids will certainly provide outstanding performance and protection.
AMSOIL V-Twin 20W-50 Works (and works very well indeed)
AMSOIL 20W-50 Synthetic Motorcycle Oil is specifically designed to withstand the rigors of engine, transmission and primary chaincase use. Expect smooth, quiet shifting with maximum component life. It is contains additives to prevent wear and the 100% synthetic base oil will not shear down (thin out). Therefore, it will not leave transmission or primary components vulnerable to accelerated wear.
Some Harley owners insist upon AMSOIL Severe Gear 75W-110 for the transmission and Severe Gear 75W-140 for the primary. But AMSOIL will tell you that there is absolutely no upside to using gear oil in these components rather than the 20W-50. Some may think that they are using a thicker oil by using a gear lube. But remember that engine oils and gear lubes use different viscosity scales to prevent confusion.
What About Gear Oil?
Let’s look at the actual viscosity numbers of AMSOIL V-Twin 20W-50 Motorcycle Oil, V-Twin Transmission Fluid, V-Twin Primary Fluid, AMSOIL Severe Gear 75W-90, 75W-110 and 75W-140. The industry standard for measuring viscosity is in “centistokes” at 100°C.
“MCV”AMSOIL 20W-50: 18.5
“MVT” V-Twin Transmission Fluid: 20.7
“MVP” V-Twin Primary Fluid: 18.7
“SVG” Severe Gear 75W-90: 16.8
“SVT” Severe Gear 75W-110: 21.3
“SVO” Severe Gear 75W-140: 27.5
As you can see, the 20W-50 motorcycle oil is relatively close to the 75W-110 in terms of actual viscosity. So those opting for a gear lube thinking that they are using thicker oil, really are not. Those using an 80W or 75W-90 in their transmissions are actually using a much thinner oil. The 20W-50 motorcycle oil’s viscosity is right in the “sweet spot” for optimum operation in engine, transmission and primary. The same applies to AMSOIL’s V-Twin Transmission and V-Twin Primary Fluids. The volumes of positive reviews we receive back this up.
Gear lubes do contain heavy doses of “extreme pressure” additives which many believe prevent transmission wear and provide quieter operation. Again, AMSOIL 20W-50 Motorcycle Oil is packed with additives specifically designed to prevent wear in clutches, gears, chains and sprockets. There is absolutely no downside to going the 20W-50 route.
Some tout the use of automatic transmission fluid (ATF) in Harley primary chaincases. We do not recommend this option. Again, let’s look at actual viscosity numbers to back this up. The viscosity of a typical ATF at 100°C is 7.5 centistokes. We submit that ATF viscosity is far too thin for this application and think that the risk of long term wear is far too great. AMSOIL 20W-50 has a viscosity of 18.5 centistokes.
Convenience: One Effective Product For Everything
The second main reason that we recommend using 20W-50 motorcycle oil in the engine, transmission and primary chaincase is that you can buy one product for everything. Therefore, you can keep one product on hand and bring one oil along on trips for top-ups. In conclusion, not only does AMSOIL Synthetic 20W-50 work exceptionally well in all three holes, it keeps things simple.
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A reader asks: A shop did some work on my Harley-Davidson and filled my primary with synthetic oil. I have always used regular H-D oil. Can I drain it and replace with regular oil? Will this cause any problems?
As you can see, the 20W-50 motorcycle oil is relatively close to the 75W-110 in terms of actual viscosity. So those opting for a gear lube thinking that they are using thicker oil, really are not. Those using an 80W or 75W-90 in their transmissions are actually using a much thinner oil. The 20W-50 motorcycle oil’s viscosity is right in the “sweet spot” for optimum operation in engine, transmission and primary. The same applies to AMSOIL’s V-Twin Transmission and V-Twin Primary Fluids. The volumes of positive reviews we receive back this up.
