Sparks valid point and I could see it on older well worn engines. Now if Big Dog is saying they don't recommend it due to leaking on a fairly new engine (say one that is just after break in), I would say they need to learn to better assemble engine and use proper gaskets, torque and sealer.
That all said, no where is my O & M manual does it say not to use Syn for Engine, though it does say BD does not recommend HD Syn for the Primary. Regardless he is talking about the primary and if he changes the primary case O-ring and inspection cover O-Ring, he should have no problem.
Not saying the tech heard wrong for there is no doubt in my mind that someone at BD told him that about the Syn, now the question is if that is a an official BD written policy for their dealers sure have no problems putting it in, at least at some. Also if I bought a brand new BD, put like 2500 miles on it and change over to Mobile 1 or other good API certified 20W-50 V-Twin oil and the cases started leaking like a sif, I would find it very hard to see how BD could blame the oil if they were to denie the warranty claim, for I think I would not have much issues bringing in experts that could say the leaks were due to bad gaskets, gaskets surface, selants, assembly and not oil.
Find it funny that S & S, HD, Corvett and hell so many others have no problems changin it over and not worry about it leaking!!!
More than likely it is an opinion of one of the tech and not based on facts or solid technical information. Granted if one is to change over to Syn on an old Toyota with 50k on it and leakage starts, then though the oil did not creat the leaks, there is leakage due to worn parts and micro gaps in the sealing surface that was being plugged with parafins and dirt and the syn clean it all out and now, due to bad gakets and sealing surface there is a leak.
If I do the same on an engine that just broke in, then I say poor assembly!
Just my 2 cents.
