but I do know there are written procedures to allow it's use in the Garrett engine
ATSB – AO-2013-183 - Investigation into an accident involving an Ayres Thrush TPE331 variant
Fuel
The operator reported that the fuel being used in JAY was a 70/30 blend of diesel and aviation turbine fuel, and included a fuel additive. The manufacturer of the fuel additive advised that the purpose of the additive was to offset the potential adverse effects of using diesel fuel in turbine engines. A fuel sample was taken from the operator’s refuelling equipment and sent to a National Association of Testing Authorities-approved laboratory which reported that the chemical composition was consistent with a predominantly diesel blend. The engine manufacturer advised the ATSB that the TPE331-5-252M engine was not approved to use diesel, nor were they aware of a supplemental type certificate allowing it. They also advised that, in general, running on diesel fuel instead of aviation turbine fuel will lead to:
• Increased carbon build-up on the fuel atomizer and combustor due to the higher distillation end point
• Increased fuel system deposit or gum formation as fuel thermal stability and gum formation are not controlled in diesel fuel
• Decreased altitude relight envelope due to lower volatility
• Poor cold weather starting due to higher fuel viscosity
• Poor cold weather operation due to higher fuel freeze point
• If the diesel was ultra low sulfur diesel it would typically have poor lubricity (unless lubricity additives were added) which could degrade fuel pump life
• If the diesel was high sulfur, it could degrade turbine coatings • Diesel fuel can contain bio-diesel…currently not approved in jet fuel
• Diesel fuel can contain any number of additives which are not approved for aircraft gas turbine engines, and the effect on the engine is unknown
In addition, the engine manufacturer stated that none of their engines were approved to use the fuel additive reported to be in the fuel blend. On 26 May 2014, CASA issued Airworthiness Bulletin 28-0155 relating to the use of diesel fuel.
This bulletin stated that the use of diesel fuel in turbine engines was only acceptable if the fuel was approved by the manufacturer and the fuel conformed to a specification detailed in their approved data.
Diesel Fuel | Civil Aviation Safety Authority (casa.gov.au)
Even though the ATSB found that the engine was producing significant power at impact, and which indicated that the engine was unlikely to have been adversely affected by the use of unapproved fuel, it seems pretty clear that Garrett doesn't approve the use of diesel fuel OR additives in its engines.