None.
You can safely reduce the feed hole in the bush to about 3/32" or 2mm (n.b. assuming you will use an in-line filter).
I've run a pressure fed engine since the early 1980's with no o-ring and no oil pressure problems.
n.b. I should probably add that it likely depends on the rest of your set-up - there's more than one way to arrange an oil feed.
You can safely reduce the feed hole in the bush to about 3/32" or 2mm (n.b. assuming you will use an in-line filter).
I've run a pressure fed engine since the early 1980's with no o-ring and no oil pressure problems.
n.b. I should probably add that it likely depends on the rest of your set-up - there's more than one way to arrange an oil feed.