"if you ride in winter that much why aren't you using … a singlespeed?!"
Because (wheel) spinning in granny gear is far easier up a steep sloppy climb than singlespeeding up it?
"where do you fit them on the cable run as you get a pack of three"
If you've not got full length outer, eg most HTs, then put one in each of the 3 sections of outer cable
If running full length outer cables then I've got one in the front mech cable and 2 in the rear mech cable
I positioned mine such that when I turn the handlebars the front and rear mech outer cables no longer touch the frame. Instead the rubber O-ring of the cable-oiler touches the frame so they make a great frame protector/bump stop for the outer cables if positioned right
"don't see the point on running them and full length cables… where is the mud going to get in?"
Mud gets in the rear mech end eventually. Used to take about 3 months for me before decent shifting went completely to pot. Once you get mud in a full length outer it seems to bung up far worse non full length outer
The cable oilers mean you can use WD40/GT85/etc to push all the mud/grit/wet out and relube your cables. I only do it when the shofting starts to become ropey (blocking up), and every time the shifting returns to being really light and easy
"the oilers are much cheaper than fancy sealed cables"
Yep. £6 for oilers + cheap cables plus cheap full length outer cable is way less than avid/gore/XTR sealed cable sets
"is it the 4mm ones for shimano gear cable? "
Yes, though some outer cable is too fat to fit. eg the Jagwire full outer that comes supplied on some Specialized bikes
" you can wrestle with the little fiddly O-ring"
I can roll them back with my fingers just fine
"WD-40 runs outside the cable outer and drips straight on to the carpet"
You thought using WD40 indoors (over carpet) was a good idea?
"so are they worth it or not"
Yes, go for it
One final thing, you'll need to use standard outer cable ends (ones that don't have elongated cable hugging tubes on the ends of them to allow WD40/GT85 to push muck out of the cables
EDIT: and another, thats a ridiculously long post. I apologise