The oil inlet thread size is where the oil will enter your turbo. There are many
different thread sizes, and some turbos aren't threaded at all. The easiest
way to determine which thread size you have is to test fit differnt bolt thread
sizes into the turbo until you find the correct one. Some standard sizes are
1/8 npt, 1/4 npt, 7/16 inverted flare, and an assortment of metric sizes usually
a 10mm or 12mm with different thread pitches.