This is pretty much an old wives tale with a little bit of truth.

Older, higher mileage motors that have been run on dino oil, have two things commonly happening in the motor. They often have sludge buildup in the motor from the dino oil, and they have motor gaskets that are getting older (dried out) and failing. Often, the only thing keeping these old gaskets from leaking, is the sludge buildup on the gasket edges.

Synthetic oil has high detergent properties, that's why motors run on syn virtually never contain any sludge.

Installing synthetic oil in an older, high mileage motor, puts the cleaning action of the synthetic to work. As the synthetic starts breaking down the sludge that's basically keeping the old gaskets from leaking, leaks can start to occur.

Did the synthetic really make the gasket leak? Not really, it just exposed a gasket that was over due for replacement.

 

If H-D thinks the oil is the cause of the problem when you bring the bike in for warranty work, they will send it off the lab and same test done to see if the oil is in spec.

 

Big Big mistake.... You do realize Harley's 20W50 is from the prime rib section of T-Rex and Mobil 1 is from some black book witches brew?

 

I believe more in samhallnc's theory more than cHarley's line. When I brought in my bike for the 1000 mile service at the HD Dealer I was asked if I wanted syn. instead of dino and I opted for the syn. (they put SE3 syn in both holes). When I did my own oil change at the 5000 mile service I decided to go with Amsoil in both holes. Shortly after that I got the "dreaded rocker box gasket leak" samhallnc refers to. Now these gaskets are not 1) real old gaskets and 2) were not gunked up at all. Yet, sure enough there is the rocker box gasket leak. I put off pulling the rocker boxes and it has since stopped leaking completely. As long as it does not start up again I won't bother changing them. I thoroughly cleaned the area of the past leak and there has not been any more for a while now, over 4000 miles more anyway. In about another 1000 miles it will be time for its 10000 mile change and I will see if it leaks again or not. Leak was minor to begin with with just enough to drip down the head fins and down onto the starter. Only a little bit of smoke from the head fins and that only lasted a couple of runs before it stopped. Never even lost enough oil to make a change on the dip stick. But it did happen with NON-OLD, nor GUNKED UP gaskets. Just saying.

 

Words of Wisdom :

Make mine "Genuine" N/S

 

No, what you got is the very common and well known problem of leaky rocker box gaskets on the 07 and later Sporties thanks to changes HD made to the rocker box and and gasket design for 07. The forum here has had hundreds of posts on the issue.

 

My 2008 sportster with 14,680 has only had the standard HD 20/50 and never has it leaked. I think if you change your oil & filter when it's due you want have any problems. And oh make sure your tires are at proper air pressures which on mine are 40 psi both ends.

 

I did the switch to synthetic "FAD" thing at 15K and leaks started at the rocker boxes. switched back to dyno oil and the leaks stopped after a coule of changes. 40K now and everything is good. So I ask again...WHY CHANGE OILS? When you buy a car is the first thing you do is change everything to something else?

 

I changed because Sportster are know to run a bit hot and the Amsoil makes my bike run about 10 degrees cooler than the dino oil does. Also the engine runs quieter with the Amsoil as well.

 

Yes, thats the very first thing I do to every vehicle I buy.....unless its new. I do not know and therefore do not trust the person I bought it from knew what they were doing.