As I read it, if it's any 'GL5' rated, they're happy since all 'GL5's should be the same. (I just went out and checked my jug of Mobil 1 75W-90, and its rated 'GL5', so I'm happy, since I've been using it forever.)
And it's available everywhere too.

 

They included Spectro with my DD6!

 

Why not run what HD recommends?

 

Because HD can't seem to make up it's mind. For decades they recommended gear oil for the tranny, 80W-90, with a recommended 5,000 mile service interval.

Then mysteriously the first "Anniversary 15th" FatBoys showed up in the '05 model year with Syn 3 in all three holes. (Remember when MoCo warned us about the evils of synthetic oil in a Harley motor back in the 90's?)

Then in '06 with the advent of the 6 speed Dyna (Guinea Pig,) the extended the service interval change to 20,000 miles.

Now they are going back to gear oil!

I trust Baker and other professionals with a vested interest in their product's longevity 10 times over any recommendation from Harley. (It's "normal"... )

 

Has anyone ever fried their TC tranny by running the recommended synthetic, from normal use?

 

The recommended tranny lube is gear oil (once again.)

 

Yes, you can run Mobile 1 in all three. Another choice would be Mobile 1 for the engine, HD Formula+ (which is also a GL1 motor oil, not a gear lube) for transmission and primary.
The above is your safest choices and motor oil is the #! choice of Mobile, Amsoil and Harley as well as Harleys Formula+ for the HD transmission and primary.

Gear Lube is a second choice and many have good luck with it, keeping in mind gear lube is/was intended as differential lube not transmission oil. Some high pressure additives of gear lube, depending on the company are not the best choice for seals and corrosion of soft metal for this reason I do not use it as there, for me personally, is no benefit.

 

If you contact Mobil they will tell you V-Twin is fine in all 3 holes.

 

But they also do recommend their Mobil 1 Racing 4t 10w40 for the primary.

 

That is true.