Hey Jay...
Does this Syn 15W-40 shear less than the 5W-40 in your bike?
I've always run regular T-4 with great results.
Never put 5w40 in any of my bikes.
I'm a long time T4 user as well.
Yes, the 15w40 should be more resistant to shearing than 5w40 or the T4.
I live in Dallas so I am more concerned with heat than cold, but the T6 15w40 is good for starts down to -22F, far colder than anything I'd ever ride in.
I have been perfectly happy with the T4 for many years (before it was called T4!), but my car greatly prefers synthetic oil, so when I found Rotella 15w40 in a full synthetic, I had to try it.
The jury is still out as to whether T6 is actually better than the T4, but I expect it will be.
While the T4 looks and feels like good ol fashioned motor oil, the T6 has this amazing tacky texture that is probably really bad for the environment... Lol
Seriously, I got a little T6 on my fingers, and it did not want to come off with soap and water. It took a serious amount of soap and scrubbing and my hand still felt kind of oily for the next 24 hours... A gallon of T6 could probably unintentionally exterminate the wildlife in an average sized pond!
Definitely wear rubber gloves.
I usually change my oil once a year or every 3k miles, whichever comes first. Last year the T4 in my Vtwin Shadow had 3700 miles on it and still looked pretty good when I pulled it out.
As far as shifting, the T6 seems about the same as the T4, but before with the T4 I was getting 52 mpg in my Shadow 750, now with the T6 I am averaging 54-56 mpg.
Going from Honda 10w40 to T6 in my CB1100, my mileage jumped from 40 mpg to 46 mpg, with an all time high of 48 mpg on the T6.
I will buy the T6 15w40 if it's available. If not, I have no problem going back to the T4.