It uses a special grease, that usually only comes with a rebuild kit. The thinking is, you don't mess with it, unless you going to replace it or rebuild it. You'll just compromise the boot trying to add grease.
If the boot is in good shape and its just a little grease, just keep an eye on it, they can leak a little and still last for years more.
If the boot is torn or cracked, replace the boot with a special kit that might NOT work, or rebuild the axle.
I'm sure there are people that have added the grease, or fixed the boots and it worked, BUT, I'm NOT one of them, hopefully one of them will come along and give you the info your looking for.
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