Well, I use Stihl HP Ultra myself and I am quite satisfied that it does the job. I am pretty sure that the Motul 710 oil would work well too. If you are concerned about the warranty then using the suggested oil is good. If that is the oil they say you should use, then it is probably best to stick with it.

But over the years I got to the point where I feel there is no warranty from the manufacturers anymore, so I usually don't bother even trying to get a warranty on something anymore. They will look for any excuse to get out of honoring a warranty problem anymore. So I would have used Stihl oil myself if I had the engine.