I use 80/90 Valvoline in mine as well, with no engaging/seal problems either. Even after a night at sub freezing temps the locker works without fail. Almost everyone I wheel with runs ARB's, and none of us have had the problems you have had.

What type of air source are you using to engage it? To high of air pressure is an easy way to blow the o-ring.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed