The spindle and apron oilers are 8mm on my 602. All the others are 6mm. The manual is wrong on this, as it is on a number of other dimensions. I suspect some production changes over the years and the manual managers not keeping up with them. Caveat emptor!
The failure mode that I experienced was pushing the spring and ball past the rather skimpy swage at the bottom of the housing. I pulled the oiler out, retrieved the ball and spring, and reinserted them in the housing, finish with a more aggressive swage. The repaired oilers haven't failed since.
If you wish to use the same style oiler, an 8mm x 7mm bushing could be turned and pressed in and the 7mm oilers used. If you have any friends that shoot a 284 Winchester, the necks on the casings are very close to 7mm x 8mm.
You should be able to get replacements from Grizzly but make sure that specify the 8mm oiler.