BSPT has 28 threads per inch vs NPT's 27. BSPT is "metric." They are both tapered pipe threads.
Unless it is very unusual fitting, it is not a 9mm thread. Sealing threads that have to seal are usually tapered. (BSPP exists, which is parallel thread).
I'm suprised your parts store guys don't know what you are talking about. This would be the same adapter for almost anyone who wants to put an aftermarket oil pressure gauge on just about any import. (Toyota/Honda rice rockets).
Will the fitting that came with the guage just barely start to thread in, but then get tight? If so, you probably have a 1/8" BSPT fitting on your engine. 1/8" NPT will start to thread into 1/8" BSPT since they are only one T.P.I. different.