Like many I've wanted to add a "Real" oil pressure gauge, but wanted to keep the Stock gauge as well. Found this adapter 1/8 BSPT x 1/8 NPT.
Scroll down a page to find the adapter.
http://208.109.215.220/files/gauge.pdf
Scroll down a page to find the adapter.
http://208.109.215.220/files/gauge.pdf