Keith and Elwood are right on the mechanical thing. Did that on the Moo boat right after I got back from Mead, (Crewcheif's advice) only to find that yeah, the oil pressure was about 115 (Prolly due to the oil I run and over building the oil system in the thing).
Fixed that with the remote dual filter ass'y and remote oil cooler, but still, good information. (My example is an extreme).
Now I run about 75-85, which is high, but manageable.
Ray
On the tach, dunno. Too much seat time in cars and boats to let a tach BS me. Double and triple check your wiring, and yeah, check to make sure it's in 8 cylinder mode. I've never seen a faulty tach, only faulty wiring, installation, or reader,

But I'm sure they exist.