Originally Posted by Yardstick
Rotella 15W40 according to the MSDS is: Highly refined mineral oils and additives.
950/990 Oil Recommendation
>= 0C (32F) 10W/50 Fully Synthetic
< 0C (32F) 5W/40 Synthetic (Rotella T6 meets this, actually -but your 15W/40 meets neither spec.)
690 Oil Recommendation
10W/60 Synthetic Engine Oil
Alternate: 10W/50 Fully Synthetic Engine Oil
Which is why if I were traveling the world I would want a bike that could tolerate a large spectrum of oils. And not just well enough to get by, but well enough to still have long oil change intervals.
The most important requirement in the KTM owners manual is that the oil meet "JASO T903 MA" specifications. This denotes the use in motorcycles with our engine types. See here
The viscosity of multi-weight oils is vs. actual viscosity is complex to be sure. As long as it isn't really cold (think near-Arctic conditions) or really hot (equatorial deserts) most any JASO MA oil will do.