I've been using Syn3 in my '04 Road King in all 3 holes for 47K miles with no problem. I shift hard under full power quite often but can't seem to find any metal particles on the trans drain plug. In most motorcycles, the oil has to do double duty in the engine and trans, and things hold together just fine - even in the hottest sport bikes. Harley trans oil stays relatively clean and cool because it never touches the engine so I don't see what the problem is. It makes no sense to me to put some thick goo in the trans where it's just going to create drag, fluid friction and heat. With that said, I just dumped my last bottle of Syn3 into the trans and will be switching to Mobil 1 in the engine and trans, and will try their 10w40 in the primary, on their recommendation.

 

You will never have a problem using regular HD oil and tran./primary fluid. That's all ya need, just keep it clean. I like to dump my oil after 3000 miles or once a year, trany/primary longer.

 

Viscosity of the 20W50 motor oil is basically the same as 75W90 gear oil.

 

These are all true statements. Any viscosity near spec (e.g., 15w50 or 20w60) will work well, too, and you can add Royal Purple and Redline to the list of qualified brands. You can change synthetic at 5k intervals with no problems, but if you run fossil oil it would be best to change every 2.5-3k.

Harley Syn3 is an acceptable product but is grossly overpriced, IMO.