synthetic oil
Although I do not yet use synthetic on my bike, my new 2002 FLHT due in Jan will live on it. I have used it extensively in other motors. The flat spot issue is an urban rumor. Some benefits are cooler running, high coaking temperatures, better film adhesion and low moisture suspension. Down fall, has a tendency to seep, and HD needs no help here, and cost. On an older engine, it tends to clean and remove crevice carbon, seemingly causing older engins to leak. The leak was already there, just pluged with gunk. Many high performance engines recommend and are factory supplied with synthetic. Some issues that have given syn oil a bad rep is allowing extended service periods with syn oil. Although synthetic oils have different moisture and acid retention mechanisms then petroleum base, it still, in my opinion, benefits from often changes. If you add the cost, over the live of your sled, of changing the oil every 2k miles, even at $4 quart, you will see its value. On my family cage, dropping the oil pan after 110,000 miles to repair road damage, the cylinders still have clear honing marks, compression at 7 pounds between highest and lowest, still runs like new and has never had a mechanical problem.