I have run both syn. and reg 20w50. I have TP rocker boxes and they have more top end noise than the S&S rocker boxes. I have less top end noise with regular 20w50. I also add about a half quart of Lucus Oil Stabalizer during the summer months. Seems to quiet them down even more. I think both oils are good and the noise is normal. Also make sure you check your oil when the motor is hot. The reading on the dipstick will be different from cold and hot. It will show more oil when hot than when cold. On my 06 you check the oil with the bike on the kickstand and motor hot. What I did was checked the oil the way the manual stated with the motor hot and then came back the next day and checked the oil with the motor cold and noticed the oil was at the bottom of the neck. Now it doesn't matter when I check the oil I know were it should be. Hope this helps.