Decalin

Apparently, yes. I have seen no actual evidence, but all of the Rotax techs that I respect say yes, definitely use it.

YES

some reading here

https://www.rotax-owner.com/en/912-914-technical-questions/4577-decalin-fuel-additive

Yes. Many have been running it for decades. Almost all the guys I know that have to use 100LL occasionally all use it. It's worth the use. It's not a cure all, but certainlly helps.

https://decalinchemicals.com/products/decalin-runup-fuel-additive/