I filled mine about 25 years ago- don't remember the oil I used, but I do remember how I did it.
I read the shop manual, the tech tips (jacking up one side of the car, loosening the clamp on the high side, et al) and thought I would try something else. The boots on my rack looked very stout. I moved the rack from side to side, noting which accordian section of the bellows moved the least. I took a big syringe with a hefty needle and poked it into the highest portion of the accodian section and proceeded to inject the oil into the rack. It took a few syringes and a little time to do. I put a dab of butyl rubber on the injection site and it bonded with the rubber. 25 years later, the boots haven't torn, the tiny hole is still sealed, and you'd really have to look to even find the spot I did the work. Kind of unconventional, but it worked for me in this instance.