How AMSOIL Performs in the Harley-Davidson Milwaukee-Eight

In 2016, Harley-Davidson* introduced its Milwaukee-Eight* V-twin engine in part to increase power and reduce engine heat. How does AMSOIL perform in this Harley-Davidson engine?

We’ll get to that soon, but let’s cover the background story first.

Harley-Davidson interviewed 1,000 riders in seven cities to find out what they wanted in Harley-Davidson’s next generation of touring bikes. Two themes emerged: more power and less heat.

Traditional air-cooled V-twin engines, like those used to power most Harley-Davidson motorcycles, can get hot. In fact, some riders have complained about the intense heat radiating from the engine while riding, particularly the rear cylinder, which doesn’t receive as much airflow as the front cylinder.

The engine also suffers from heat

Extreme heat not only challenges rider comfort, it challenges the engine.

If, for example, extreme heat causes the piston to expand beyond acceptable tolerances, it can rub against the cylinder wall and cause scuffing.

Heat also causes some oils to thin and lose viscosity. The oil can become so thin the engine loses oil pressure, causing the oil-pressure gauge to bottom out.

The rider may notice increased valvetrain and gear noise as parts clatter together. Any rider worth his or her salt knows you don’t run an engine with no oil pressure, so he or she shuts it down until the engine cools enough to restore oil pressure.

AMSOIL vs. Harley-Davidson Oil: How We Perform

Air ≠ liquid

Why do air-cooled V-twins get so hot? And how hot can they get?

It comes down to physics.

Air-cooled engines rely on air flowing over the engine to remove heat. In contrast, liquid-cooled engines, like those found on modern Indian* motorcycles, use a coolant/water mixture to absorb engine heat and carry it to a radiator, where it dissipates into the atmosphere. 

Air is inherently less efficient removing heat than coolant/water, meaning your average V-twin powering a Harley-Davidson runs hotter than a liquid-cooled bike.

Riding conditions turn up the heat

Your riding conditions also affect engine heat.

Have you ever gone to Sturgis? Or ridden in a Fourth of July parade or other event at the height of summer?

In Sturgis, riders crawl down Main Street, taking in the sights and sounds. The August sun beats down on the asphalt and radiates onto the bike. Traffic can creep along so slowly that air doesn’t rush over the engine like it does when riding on the highway.

This lack of cooling air moving across the engine can cause heat to climb to dangerous territory.

If it’s bad enough, the electronic temperature controls that safeguard the bike against overheating can trigger, forcing the rider to pull over and stop until the engine cools.

The Harley-Davidson Milwaukee-Eight runs cooler

I’m no marketing genius, but sitting alongside the road waiting for your bike to cool isn’t good brand strategy. So, the engineers at Harley-Davidson designed the Milwaukee-Eight engine in part to run cooler.

Right on Harley-Davidson’s website, the company presents the Milwaukee-Eight as the “…most powerful, coolest-running motor we’ve ever built.”

More power, less heat. Check.

AMSOIL and the Harley-Davidson Milwaukee-Eight

We bought a 2019 Harley-Davidson Street Bob with a Milwaukee-Eight 107 engine for extreme-heat testing. We wanted to see how AMSOIL 20W-50 Synthetic V-Twin Motorcycle Oil held up in Harley-Davidson’s newest engine.

Prior to testing, we instrumented the bike and rode it on the streets to determine the operating conditions that would most severely challenge the bike and the motor oil.

As you might expect, stop-and-go, slow-moving traffic increased heat and presented the most severe conditions, similar to the conditions you see at a bike rally on a hot summer day.

So, our engineers designed a 1,000-mile dyno test meant to replicate the worst-case scenario: slow-moving operation in extreme heat with little-to-no cooling air passing over the engine. 1,000 miles is enough to make 640 trips down Main St. in Sturgis.

How does AMSOIL perform in the Harley-Davidson Milwaukee-Eight?

The test created conditions that held the oil-sump temperature at a steady 300ºF (149ºC) for 1,000 miles. The rear-cylinder temperature surpassed 420ºF (216ºC). That’s hotter than your bike should ever get.

Heat that extreme chemically breaks down most oils, causing it to lose viscosity and fall out of grade. It can also lead to oxidation, which hastens formation of harmful deposits that can cause the rings to stick, reducing compression and power.

After 1,000 miles of extreme heat, AMSOIL 20W-50 Synthetic V-Twin Motorcycle Oil delivered excellent protection.

Compare a brand-new rear-cylinder piston before the dyno test to one following 1,000 miles. It contains minimal deposits, the rings are free and it’s in great condition.

Chemical analysis following 1,000 miles at 300ºF (149ºC) shows the oil maintained viscosity and virtually prevented wear.

Check out the video to see the full results.

Video transcript: V-Twin Extreme Heat Torture Test

What good is all that power if your oil can’t handle the heat? Extreme heat can cause oil to lose viscosity and become thinner, reducing protection and leading to poor shifts or compensator noise. We formulate AMSOIL Synthetic V-Twin Motorcycle Oil to fight extreme heat and stress in the toughest conditions. In fact, industry standard testing shows that AMSOIL resists viscosity breakdown six times better than Harley-Davidson* SYN3* for improved protection against compensator and transmission gear wear.

Protect your bike in the spirit of the American biker. Upgrade to AMSOIL protection at amsoil.com/vtwin.

So, does the Milwaukee-Eight run cooler than past Harley-Davidson V-twin engines? 

We didn’t run a side-by-side test against an older engine, so we can’t say for certain. But Harley-Davidson designed the Milwaukee-Eight to run cooler, and it appears the company succeeded.

That’s thanks to a series of oil-filled cooling lines that circulate around the cylinders and carry heat to an oil cooler.

The oil in your bike should never get this hot. But, if it does, AMSOIL Synthetic V-Twin Motorcycle Oil has you and your Milwaukee-Eight engine covered.