MILSPECs are designed around the applications and operating conditions. I vaguely recall from my "Motor-T" days that there was one spec for normal-to-hot conditions and another for very cold. Since my unit was in Florida, we didn't worry much about which spec to apply, as to oil anyway. At the user level, the mech puts in whatever gets shipped over from the warehouse. It's up to the folks who run the supply chain to determine which suppliers will be awarded contracts to provide the military anything -- oils, bandaids, ammo, flu vaccine, whatever.