Beg to differ. The Zerk end is standard, but that end which threads into the tractor, which is what 1/8 NPT would reference, is indeed metric, 6mm as I recall.
Well, you're both right. Those on the Yanmar tractor and factory implements are metric, those on the additions such as an FEL, back-hoe, post hole digger, rotary cutter etc will most probably be US standard fittings.
I have 8mm zerks on my fel and have broke a few cuz the grease gun latches on to them so tight... The zerks on the tractor I have not had any probs with so I don't know what size they are. Folks here say 6mm.