I've been using Mobil DTE Light for several years. Wren approves its use in their engines. It is used in some full-size gas turbines that are ground based e.g. power generation, as they don't need the additives for very high altitude flight and neither do we. I believe it is also free from the very harmful organophosphates that aero turbine oil contains. Without carriage it comes to about £4/litre. Since it only comes in 20 litre container, share it with some other jet fliers.
H