What did you end up using?
I have to do the service on my Massey and need to buy fluid ASAP.
I was interested in the Amsoil 5w-30 hydraulic fluid too, but the only experience I have with it is in the hydraulic steering systems of off-road vehicles (using full-hydraulic steering) in extreme conditions, and it holds up well to heat.
Avalanche 8.1L- RS T6 5-40/M1
Modded '98 TransAm- M1 5-30EP/M1-206
Aprilia RSV & Tuono- RS T6 5-40/Hi-flo filtro
Subaru XV Crosstrek- M1 0-20/Wix 57830