990's are broken in at the factory with Motorex Power Synt 10w-50 full synthetic. Then KTM refills them with the same thing and delivers to dealers. The engine was born with full synth in it so I'd keep running it. And actually, these motors are designed for full synthetic. All oil belief structures aside, it's a good idea to run the type of oil that the engine was designed for: 10w-50 full synth.
I was told KTM uses Motorex Power Synt 10w-50 full synthetic in this machine:
Tried to embed the YouTube video into this post but admins must have disabled embedding of video in vBulletin or something.