Bumping up this thread with more information.
I tempted fate too much saying my oil filter cap didn't leak. After the last oil and filter change it started to weep. I lifted the cap and fitted a new filter and seals. I found, as said further up the thread, that the O ring was distorted, there was also a couple of scratches in the groove where I or my predecesors had been clumsey getting the old O ring out with a sharp screwdriver. One side of the O ring was thicker and the opposite side visibly thinner. As threatened I measured everything up. The filter housing has a 100mm bore, the groove in the cap seems to be 92mm (verniers wouldn't quite go over the diameter) and the new O ring was 92mm ID and 4mm cord diameter. The old O ring was down to 3.5mm on the thin side and squashed to an oval shape on the thick side. The new O ring has done the trick, as tight as a nuns bottom again. For information the smaller O rings on the stalk are 6mm ID by 2mm chord diameter and 10mm ID by 2mm chord diameter.
I've never looked closely at the replacement O rings before fitting, I will now. I'm also getting some spare ones, they're available for pennies on the Bay of E.
So the moral is; the O rings in the filter boxes may not be fantastic, look closely before fitting and keep your old one till you know the new one is leak tight. They don't age very quickly and could be used again if you get a poor one.
This is all related to the 99MY later type, I don't know if the earlier filter had the same seals.
And I'm still using my chain wrench just above the joint, the hex hasn't got a top and the hollow hex looks very femmer.