Folks often come into r.m.h asking about synthetics, and nearly always come with a story related to them by some mechanic, or a "friend of a friend" urban-legend-like horror story. Sometimes they're afraid that using any oil other than Harley Davidson's own oil, let alone a synthetic oil, will void their warranty.

Synthetic Oils are too "slippery"
This is one of the most pervasive myths ever conceived. It is a myth which is promoted in an oral tradition that dates back to the first "tall tale", and like most myths, contains a kernal of truth.

The origins of this myth are, like most myths, unclear. It appears to have started with users who ride motorcycles that use the same oil for the engine, clutch and transmission. Some riders reported (and still report) that their clutch slips when they use some brands of synthetic oils. It's entirely possible that the lubricating and anti-wear qualities of some oils are so effective, that they also want to limit the friction between clutch plates. So it's possible that some oils really are too slippery for that type of application.

All Harleys use a separate oil supply for the engine and the clutch. In the case of Big Twins, they also have separate lubrication for the primary (where the clutch is bathed in oil) and the transmission. So this is a non-issue where Harleys are concerned.

The "too slippery" argument is used by nearly all manufacturers which sell their own oil, as well as makers of motorcycle oils.

It's unknown whether a different type of "too slippery" argument came directly from Harley Davidson or has just been handed down in an oral tradition on its own.

The Harley version arrives in r.m.h when someone posts: "The mechanic [service writer, parts counterperson] at my local Harley dealer told me synthetic oils are 'too slippery' for Harleys. He says it'll make the roller crank bearings skid, flatten out and make the bearings sieze."

Well, this is an interesting story. Is there truth behind it? Well, there's never been any proof offered of a bearing failure due to oil that reduced friction so much that the bearings or races skidded. Are there apocryphal stories? Sure. People have claimed that customers with hopped-up engines running automotive synthetics have come in with failed roller bearings and the post mortem revealed that the bearings were flattened. With hundreds of people posting r.m.h over the years, none have had a bearing failure that could be attributed to use of an automotive synthetic.

Now, without correct forensic analysis, one could come to the conclusion that the bearings were not turning and that this was caused by lack of friction. Well, synthetic oil is good, but it ain't that good. More likely, the bearing failed and the higher volitization temperature of the synthetic oil, and perhaps superior boundary lubrication that persists even after the oil has been vaporized, permitted the inner race to continue rotating on frozen rollers. And when the oil was finally vaporized, metal to metal contact flattened the rollers and heated them until the bearing could no longer allow the inner race to turn.

An interesting twist on this myth is that an automotive synthetic 15w-50 or 20w-50 will somehow promote this sort of slippage, but those who in the past might have argued against synthetics in general now feel that Mobil 1's "V-Twin" specific 20w-50 somehow avoids this slipperiness issue. Without any proof other than the fact that the oil says "V-Twin" on the label.

Amsoil has a nice little article about the roller bearing issue on their site. While they specifically state that there is no problem using their oil for Harleys, they also imply that they see no merit to the claim in general:

Amsoil article on the Harley Roller Bearing Issue

Synthetic Oil Causes Leaks
The source for this story is often someone who has switched to a synthetic oil after long term use of a traditional dinosaur oil. See Leaking.

Synthetic Oil Makes Bearing Cages Separate From The Bearing
This one comes not from an r.m.h poster, but from David Canitz, Technical Services Manager at Royal Purple, LTD (maker of another popular synthetic commonly used in motorcycles):

Most recent one (myth) that was told to one of our industrial client's (an
engineer) who uses our industrial oils in all types of rotating
equipment (pumps, compressors, gear boxes, blowers, etc) and also owns a
Harley was that if you use a "synthetic oil, it will make the pressed in
bearing cages separate from the bearing. No lie, this was relayed to us
as coming from the "Harley Technician" at a local Harley dealer.

Synthetic Oil Is Too Slippery For Break-in
This one might not be a myth. At least, the concensus is that using synthetic oils during break-in is at the very least a waste of money, and at worst doesn't allow the rings to wear down cross hatching in the cylinders.

Air-Cooled Engines Require a Special Oil
A more interesting point is always brought up: that somehow an "air cooled" engine requires some type of different oil than a non-air-cooled automobile. The claim is ridiculous for a lot of reasons, most notably that if the oil is run hotter as a result of air cooling, you want an oil that withstands heat even better than a conventional oil. But even this claim is untrue, as one r.m.h denizen pointed out. He used an infrared thermometer to measure the crankcase in a water cooled automobile, and it was over 300 degrees F. Cars with turbo chargers can spot heat oils to 1,500 degrees F.