Murta, many moons ago I was involved in supply of Shock absorber oils./fluids
Viscosity as you say is of prime importance but viscosity loss if VI improvers are used is significant.
Foaming too you rightly identify is a much more complex issue with solubility of gas in the base oil important.
Foaming is certain to take place also.
If I remember the only way to test shock absorber fluids is by rig testing in the shock absorber.