The concensus seems to be that it's best not to break-in a Harley using synthetic oil. As a superior lubricant, quality synthetic oils will presumably extend the break-in period and some say it might not ever let the engine break-in properly.

There isn't, however, a concensus on when break-in is actually completed. Some prefer to wait 5,000 miles to the first synthetic oil change, others change immediately at 1,000 miles. So far, there haven't been any ill first hand reports from those who changed at the earlier interval.

In any case, nearly everyone seems to agree that use of a synthetic prior to 1,000 miles is a waste of money.

What criterion should you use for adopting the use of a synthetic after the first 1,000 miles? It could depend on how you plan to use your bike. If you're planning on a 400 mile a day run through the Arizona, New Mexico or Texas deserts with temps of 110 plus degrees, the high temperature durability of synthetic oil may well outweigh other less-proven mileage based factors in the decision making process. Several oil manufacturers claim that their synthetic motor oils provide better heat transfer to the oil and thus help the engine run 5-20 degrees cooler. Since the parts are still tight, superior lubrication under extreme heat and mileage conditions might prevent excessive and unwanted wear.

If, on the other hand, you're planning on taking shorter trips in temperatures under 80 degrees without excessive traffic slowing the air flow around your bike, there shouldn't be any problem running HD or other good dinosaur oils for the first 5,000 miles. This is one of those religious type of arguments that really have no science available to back them up.