We are talking about removing seal #8 and #23 in your second image. This means that your wheel bearings, swivels, and diffs will all run in common oil.
Oil lubricates better than grease, and as Dougal mentioned is better when water ingress may be an issue.
When I adjust or install wheel bearings, I put a small amount of grease on them for initial lubrication, but this is washed away quickly by the oil.
Why were you told not to???
When we did a friend's 90 we went the whole hog and installed earlier type swivel housings with drains. But you could just as easily drill and tap drain holes into the later type.

