T5 15w40 did hold shift quality for 3700 miles for me. With that said T6 15w40 versus T4 in15w40 should hold shift quality longer, and by the numbers, shear less with respect to T6 being synthetic all else equal.I agree that Shell knows oil... My personal experience is that T4 15w40 is a superior performing motor oil in my Honda motorcycles even compared to Honda brand dino oil. I haven't tried Honda's synthetic oil yet, probably never will.
My main curiosity with this thread is whether the T6 15w40 will outperform the same grade T4 15w40 in terms of resistance to sheer, high temps, extended OCIs, and cold starts.
I haven't had a problem with the T4 15w40 for cold starts as low as 32F, but I imagine the fully synthetic might just be a little smoother for cold starts, and give me a little more cushion on the OCI too.
I will likely still change the oil once a year, I'm lazy, but I like having a little extra cushion if I can get it.T5 did hold shift quality and stayed in grade. In my bike those things are pretty much synonymous. If it goes out of grade shift quality falls off if it is a x40 to start with. 20w50 conventional or synthetic has not fallen off in shift quality however Castrol 20 W 50 conventional
Conventional oil doesn't hold up as well for my bike and that should be pretty applicable to most shared sump bikes. Castrol 20w50 conventional was in the 40 wt range after 2700 miles, but shift quality was still good from what I remember. Valvoline 10w40 ATV oil was into the 30w range in just 1400 miles, and shifting had fallen off noticeably.
M1 20w50 and 10w40 4T are good for 5000+ miles in my bike and stay in grade. So, those are my go-to oils.