The linear rail blocks have a series of port options on them when manufactured, typically one is open inside and comes with the small bolt plugging it. That’s the one you want to oil through ideally.
Some of the other threaded holes, the ones with a blind end are also options for oiling but need to be opened up in order to use them.
Here’s what my HiWin blocks look like inside - To save @Luke the trouble - NO please do not pull your linear rail blocks off the rail or open them up. I had some spares from my own rail conversion, thus I was happy to play ‘200 bearing pick up’ for re-assembly, I also had the plastic keepers that go inside when they’re not on the rail.