Too many dealers try to scare the customer/owner with a threat that if you use anything but their Syn 3 oil that it will void your warranty.
This is a bunch of lies.
Here's a letter from the MOCO that states so.
This letter was signed by Lisa Sagen, Powertrain customer service.
Also refer to these posts too...
Dealers And The Magnusson-Moss Act - Harley Davidson Community
Warrantee Denied/What To Do - Harley Davidson Community
Here's a better copy from the original letter.
Harley-Davidson Motor Company
3700 West Juneau Ave.
P.O. Box 653
Milwaukee, WI 53201
414/342-4680
January 31, 2003
Aaron Scanlon
AMSOIL Building
Superior, WI 54880
Dear Mr. Scanlon:
Thank you for your recent letter requesting information on Harley-Davidson's one-year, unlimited mileage warranty when Synthetic Lubricants are used.
Until now, the Motor Company has never supported the use of synthetic products in our vehicles because there had never been any test validations completed on the numerous formulations in HD/Buell motorcycles to determine the viability of using these products on Harley-Davidson products until now.
We have developed a specially formulated synthetic fluid called SYN3. This fluid can be used in Harley-Davidson engines, transmissions and primary chaincases. This product had been exclusively designed for Harley-Davidson and is the only synthetic product tested and certified by Harley-Davidson engineering for use on HD/Buell motorcycles.
The use of a different brand of fluid does not void the one-year, unlimited mileage warranty unless it can be confirmed that the use of such products was the cause of the failure.
Thank you, Mr. Scanlon, for taking the time to write to Harley-Davidson and allowing me to answer your questions.
Sincerely,
Lisa Sagen
Powertrain Customer Service
This is a bunch of lies.
Here's a letter from the MOCO that states so.

This letter was signed by Lisa Sagen, Powertrain customer service.
Also refer to these posts too...
Dealers And The Magnusson-Moss Act - Harley Davidson Community
Warrantee Denied/What To Do - Harley Davidson Community
Here's a better copy from the original letter.
Harley-Davidson's response to claims regarding warranty claims and voids for non HD oils used.
3700 West Juneau Ave.
P.O. Box 653
Milwaukee, WI 53201
414/342-4680
January 31, 2003
Aaron Scanlon
AMSOIL Building
Superior, WI 54880
Dear Mr. Scanlon:
Thank you for your recent letter requesting information on Harley-Davidson's one-year, unlimited mileage warranty when Synthetic Lubricants are used.
Until now, the Motor Company has never supported the use of synthetic products in our vehicles because there had never been any test validations completed on the numerous formulations in HD/Buell motorcycles to determine the viability of using these products on Harley-Davidson products until now.
We have developed a specially formulated synthetic fluid called SYN3. This fluid can be used in Harley-Davidson engines, transmissions and primary chaincases. This product had been exclusively designed for Harley-Davidson and is the only synthetic product tested and certified by Harley-Davidson engineering for use on HD/Buell motorcycles.
The use of a different brand of fluid does not void the one-year, unlimited mileage warranty unless it can be confirmed that the use of such products was the cause of the failure.
Thank you, Mr. Scanlon, for taking the time to write to Harley-Davidson and allowing me to answer your questions.
Sincerely,
Lisa Sagen
Powertrain Customer Service