I used the Mobil 1 Automotive 15/50 for 5000 or 6000 miles in my last bike and didn't like it - I heard more valve train noise.
I don't know how much difference it makes in reducing top end noise and overall performance but automotive oil is blended differently than Motorcycle oil- there are different additives put in each type of oil- and from what I have read, in simple terms is that automotive oil is blended for fuel economy, and to protect emission controls it is generally lower viscosity, where motorcycle oil is blended with different additives for durability, hi rev, high temp, severe duty.
So if you are interested: I found this is from the Mobil 1 website:
http://www.mobiloil.com/USA-English/MotorOil/Oils/Motorcycle_Oil_FAQs.aspxIt's a little hard to generalize about the difference between Mobil 1 passenger-car motor oils and Mobil 1 motorcycle oils. That's because not all viscosities of Mobil 1 passenger-car oils have the same levels of zinc and phosphorus, and there are even greater differences among the three Mobil 1 motorcycle oils. In general, Mobil 1 motorcycle oils have:
* Additive packages balanced differently for motorcycle engine and transmission operation. For passenger vehicles, fuel economy and emission system protection are higher priorities. These require low phosphorus systems and the use of friction modifiers. Motorcycle oils do not require friction modifiers for fuel economy and for better clutch friction less/no friction modifier is optimum. Motorcycle oils allow the use of higher levels of antiwear additives such as ZDDP (phosphorous).API service ratings tell the details about the oil which you can find on the back of the bottle. I don't see anywhere in the owners manual where HD states the API rating required for their engines, just to use the HD oil....or if you cant find SYn3, then use a Diesel oil- (which contains more additives to protect from viscosity break down against hi temp- hi rev/speed operation.
Here are a few more links if you want to know more about API service ratings:
http://motorcycleinfo.calsci.com/API.html from the Amsoil website:
http://www.performanceoiltechnology.com/apiserviceclassifications.htm