The mechanic at the Honda dealership in Brunswick several years ago told me that i could double the change interval if using synthetic oil. I have also read several articles that oil changes can be extended, one stating up to three times if using quality synthetic oil. I have doubled the interval on my Toyota Corolla with no problems. I do check the oil on a regular basis to insure the color has not changed. 100,000 miles and no issues. I double the interval on my motorcycles with oil filters, but probably will not on my Rebel since it has no filter. The owner's manual calls for oil change every 4,000 miles, and I may extend to 6,000. I have 3,000 on the oil in my Rebel now and it looks new. I do not abuse my bikes or ride in dusty conditions. If I did then I would not double the intervals.
2004 CMX 250 C Rebel--for the fun of it
2001 Honda 750 ACE--for the long rides
2004 Honda Aero 750 (Wife)--what can I say
1985 Yamaha XJ 700--for the old times