You can buy MIL-H-5606A from www.skygeek.com also from aircraft spruce and specialty, www.aircraftspruce.com they have good online stores and prices are reasonable. It has been around a long time and is still used. If you can’t find it under the mil spec you can find it often as Royco 756. It is a mineral based oil. It will eat rubber that is not “spec”ed for exposure to this type of fluid.