AvGas

John. NO WAY! The octane rating of leaded fuels including Avgas (100/130 or 115/145) is enhanced by the addition of tetraethyl lead. As with normal leaded fuel, use of it in an unleaded motor will lead to the destruction of the catalytic converter.
It is quite OK to use in old lead motors - have done so myself many times with helicopter fuel in old Toyotas.

Tuco

John,

As Tuco said, it shouldn’t be used, and to add to the potential damage you can guarantee that you will destroy the oxygen sensor that feeds data to your EFI system. Assuming you have one that is!

AVGAS will actually run your truck to get you out of life threatening situations, but beware of the consequences.

John,
did gunbarrel last July and needed to put avgas in 1992 80 seriesl/c petrol(unleaded).
Ran a 50/50 mixture with no problems. In fact seemed to clean out injectors a bit a first .This was at start of 13,500 km trip and vehicle ran perfectly.No noticable difference experienced in fuel economyVehicle
had 300,000 on clock at start and has done 20,000 since.
In my opinion and experience nothing to worry about.
regards
Howard