Here's a test that you syn 3 users can do to tell you something about syn 3 in the trans. and how well it is serving you.

When you drain the trans oil (syn 3) look at the magnet on the drain plug and see how much ferrous metal is on there in the form of fuzz. If you want proof once and for all how well syn 3 lubes the trans, switch to Mobil 1 75/90 or redline shockproof heavy this oil change and next oil change notice how much fuzz is there then when you dump this out next time. Granted there will be some left in the case from the last oil change using the syn 3 but there will be almost nothing. NEXT oil change after you have run the better oil in the trans for a few thousand miles, drain it again and see if you find anything on the plug magnet at all. I've done this and one thing I know for sure is that less fuzz = less wear. It's GEAR MATERIAL you see there. Would you be concerned at all to see this much fuzz in the engine? Why is it OK in the trans?

There's been a number of transmission bearings going bad all using syn 3 oil and shifter forks blued because of poor lubrication with syn 3 . Your bike your choice here.

The 06 bikes no longer advise use of Syn 3 in the trans from the factory in CVO bikes or at most dealers.

They recommend the formula + for the primary and the transmission now, but IMO, it has to fall short in one area, I feel the transmission because of the severe service encountered with the gears just as the Syn 3 did when used in the transmission.

The dealers are pushing syn 3 because they sell it and make money doing it. Your warrantee will be over in most cases when the trans has problems, food for thought .

I may be switching to Amsoil or Mobil 1 75w-145, but I have a friend with 33,000 on an 05 Duece. He has used nothing but Syn 3 the entire time, bike is in great shape. He is a former HD mechanic.

If your bud is a former HD mechanic, he should know that syn 3 is not only NOT a full synthetic but a group 3 oil which is barely better than dino oils. Besides that the 20/50 weight is light for the transmission and it is not a gear oil either despite the rating system being different than the gear lube. There are a number of differences in gear oil that differentiate it from an engine oil. The shear factor of a gear lube is what you should be concerned about in the trans and not the simplistic outlook of one oil fits 3 holes as HD tries to instill on people.
It's a compromise at best in two holes.

True, his trans will work now but will it last as long as if he had used a gear lube?
I doubt it very much.
I have a few buds that work in HD shops and have seen the insides of the trans that run syn 3 with the blued shifter forks and bearings that failed.

Here's a test you can do if you choose to switch to a gear lube from syn 3.
Drain the trans, clean the magnet on the plug and notice how much metallic fuzz is on the plug. Refill the trans with a quality gear lube and next service notice how much less fuzz is on that plug this time as compared to last time with the syn 3. That fuzz is gear materials. Not a good thing at all. I've done this many times when switching friends over from syn 3 and it's amazing what you learn from the small things that usually get overlooked.

Here's a bit more reading on gear oils and a better choice than the two you mentioned. I used the Mobil1 75/90 and 75/140 in mine until I tried the Spectro in this post. There's no comparison.

http://www.hdtalking.com/oil_related_tips/1238-oil_suggestions_for_harleys.html#post9723