I stopped using Rotella when they reformulated it, but if I were to use now, I'd go with the 15-40 stuff. The "synthetic" stuff is group III dyno, that's all. The 15-40 cheap stuff also has a little more zinc, could help your tranny/shifting.
As for M1 5-40, it is worth noting that there is another version which is much better, but cost more. I think it is called Mobil 1"european formula" and the only places I know that sell it are Pep Boys and Dodge dealerships.
It meets the requirements for a lot more engines (mostly diesel) that the regular M1 5-40 truck/diesel stuff you see. It is also the only VW approved TDI oil that is 30w (most oils approved for TDI's are 5-40)- gives me an extra 3 mpg.I stopped being a M1 fan years ago with the switch to group III, then this awesome stuff came out. I haven't tried it in my bike and likely never will as it is $&T#@&^ expensive (almost $10 a quart) and I can get Redline 5-40 motorcycle oil for $7 a qt. But hey, if you see some that is accidentally priced the same as regular M1, pick it up!