I seem to remember a similar discussion at my flying club about JetA and Diesel being used in compression ignition engines. The problem was sort of reversed because JetA is less viscose than diesel, but the answer we came up with is:

If in doubt phone the engine manufactures, and ask one of their engineers. If that draws a blank use JetA with no diesel.

It's better to pay a few more pence on the litre for fuel than to have a melt-down inflight!

WF.

I was using normal unleaded from a car garage. The Subaru engine (EA81) is an old engine so I assume it was designed to run on the old 4 star. However, the engine has been somewhat modified to increase compression, amongst other things I don't understand. EGT temps seem pretty stable at 1400°F, which is similar to what I got with the unleaded. Max temp from the engine manual is 1650 °F, so it is well within that.