Is Decalin Additive Allowed?

As I recall Decalin is not prohibited by Rotax.

Not prohibited, but keep in mind that  Decalin / TCP are NOT "officially tested, approved, or endorsed" by ROTAX.

from SI-912-016 ( latest version)

5.2) AVGAS fuel additives

Additives under the names of Decalin® and Alcor TCP®, which aid the scavenging of lead deposits have not been tested by BRP-Rotax.

Field experience shows that these products have no detrimental effect on the engine, when used in the recommended manner. Always follow the additive manufacturers instructions especially with regard to health and safety precautions.

BRP-Rotax only has field experience with Decalin Runup® and Alcor TCP® brands. Other similar additives are not recommended as BRP-Rotax cannot comment on their suitability for the 912 i, 915 i, 912 and 914 Series engine types.

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However, even though there has been no official testing by ROTAX - it has become common knowledge in the industry that Decalin/TCP have actually proven to provide a huge benefit for the engines; the evidence for that is clear after many years of use in the field.

Have a look at the links here...

https://www.rotax-owner.com/en/general-tech-discussion/6787-decalin-fuel-additive-lead-scavenger

It sort of depends on your philosophy: Is everything, that is not specifically forbidden, allowed?  Or is everything, that is not specifically allowed, forbidden? :)

Tyler Queried:

...is everything, that is not specifically allowed, forbidden?

- - -

These are Uncertified Engines in Experimental aircraft.

Just because something is specifically forbiden and not allowed does NOT mean you can't do it! 

As a famous man once said, "Son, You're on your own!"