 Re: Low oil pressure? No, bad sender!
One more comment on the matchup of U.S. and Yanmar/British 1/8" pipe thread: I tried threading a half-dozen different US-standard brass and iron-pipe fittings into the Yanmar block, which has British Standard pipe thread. Some would thread in several turns by hand but some of the brass fittings wouldn't catch the first thread. Completely random.
It appears that the new and old oil senders, which have the proper Yanmar/British Standard/Japanese-Automotive pipe thread, have a greater taper and are smaller diameter for the first few threads compared to US fittings which may or may not be too large diameter to catch the first thread. For the several brass adapters that I threaded in and tightened, their threads were not distorted when I removed them so apparently that overall taper is the only difference.
Note these comments are all about the the oil pressure sender. Yanmar used Metric threads for the temperature sender. | 
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