There are lot’s of different fittings available – all car manufacturers use metric system, but main motorsport fittings come in metric and imperial thread pitches.

AN fitting thread size and internal diameter

2AN – 5/16-24 UNF – 0.125″ (3mm)
4AN – 7/16-20 UNF – 0.22″ (5.5mm)
6AN – 9/16-18 UNF – 0.34″ (8.5mm)
8AN – 3/4-16 UNF – 0.44″ (11mm)
10AN – 7/8-14 UNF – 0.56″ (14mm)

General rule of thumb – turbo oil feed uses 4AN, oil drain uses 10AN (12AN if available)

1/4″ NPT = 1/4-18 NPSF – tappered