Wanted a screw on pour spout as well as the funnel, Found a spout that looked the part, cut in some threads, two parts to print, the cap file should be printed as is, with a nice brim, I went inside and outside on that one. The spout i used a brim but you probably don't need to.

I did NOT size up or down either thread parts on the two pieces, my printer is pretty well dialed in and i got it together with the side cutters as shown, once I put in in and out several times to clean the threads I then heated the inside of the cap (small threads only) with a lighter and then RIGHT AWAY screwed the spout in the rest of the way until flush and snug up top, it won't be 100% up to the top but very very close.

you MAY need to scale down the spout to 99% maybe 96% depending on your printer and how your settings work out.

Don't scale (try not to) the cap as the threads may then not fit on the oil bottle!!!

I am going to mix some resin and coat the inside of the spout with it, just so no bits of plastic break off and get in the oil!