Decalin

See Section 5.2:  https://www.rotax-owner.com/pdf/SI-912-016-R3.pdf

As for proportions, I use it as specified on the DECALIN container:  https://decalinchemicals.com/products/decalin-runup-fuel-additive/

I have a 20 gallon fuel tank in my plane. I use 91 oct non ethanol fuel at my home base. While traveling if I want to add 10 gallons of 100LL to my tank that still contains 10 gallons of 91 oct non ethanol fuel do I add the quantity of decalin for 10 gallons of av gas or enough for 20 gallons of gas?

The auto fuel will help dilute the 100LL. So if you do add the Decalin just add for the 10 gal.