If I remeber right the oil seal is to prevent oil from the diff casing entering the swivel, it doesn't work the other way quite as well so don't assume it's f***ed just yet, it can easily spread to allow the grease through.
Also, the oil will turn dark grey anyway if it's been in a while, it picks up the metal from the diff as that wears.
The swivel grease won't do any harm whatsoever, it's designed for a much harsher environment than the diff anyway, probably do some good. Originally both swivels and diff ran on EP90 anyway
Is it leaking to the outside, if not stop panicking.
Changing the seal can be done by pulling the complete swivel from the axle and prising the seal out, along the half shaft and removing. It's located in the axle end of the swivel ball itself. If doing the swivel seals I'd do the axle seals but TBH not the other way round, if the swivels are ok, leave well alone.