Now made from some synthetic something or another, but yes, used to be whale oil derivative.
Lifespan of the additive in a posi seems to be in the 50K range, so your situation was pretty normal. Gives just enough slip to the clutch facings in slow speed applications to avoid grab/sudden release on the packs - that's the noise you were hearing.
I have cars with both GM and Ford LSD's and use the mfg recommended additive for them both. I also change rear gear fluid every 30-40K miles or so and always add the 4oz additive. IMO that waiting to 150K miles is too long on any gear oil, dino or synthetic.
For anyone with the Ford 9 rear axle, to avoid having to pull the axles and center to drain, or have to use a siphon, I weld in a bung at the bottom of the housing for an allen plug. Easy drain. This is easiest on a new installation, but can be done on an existing housing in the car - but you gotta pull that center to drain the fluid and get the plug welding done.