There's nothing "metric" about it and you'll also never find anything metric specified in fractions - the oil sender port is 1/8" BSPT - british standard pipe tapered - it looks very similar to a 1/8" NPT (national pipe thread) but it is very slightly larger in diameter, and also has a closer thread pitch (27 tpi vs 28).
You cannot screw a 1/8" NPT male fitting into a 1/8" BSPT female port without cross threading it, but you can in most cases get a 1/8" BSPT male fitting into a 1/8" NPT port and get it to seal because of the taper on the BSPT fitting.