it's 1/8" British Standard Pipe Thread (bspt)

go to http://www.z10eng.com/

under block fittings, and you will find an 1/8" bspt to 1/8" npt adaptor fitting, this will allow you to use any combination of Ts and other standard fittings to adapt whatever you want to adapt to the oil pressure sending port.

BTW, I just used a standard 1/8 NPT fitting, but you do have to bury it deeper than the proper fitting, but 3 years no leaks....