I have heard of lube oil additives to stretch out oil life. while this maybe okay with low pressure, moderate temperature oils, If the oil is also shared with the high pressure system, such additives will cause adverse properties such as accelerated changes in viscosity, greater varnishing potential, separations issues. etc... The best way to stretch out oil life is to ensure its free of water, ensure particle count is low, maintatin your filtration system, and possibly periodically running the oil through an extrenal cleaner/electrostatic and particle remover.
thanks