I would replace the sending unit anyways, even though your Mechanic said it was OK. I don't know of too much else it could be unless it is in the gauge or a ground or something.
My oil pressure gauge gets pegged under throttle when the engine is cold. (The gauge goes up to 80psi) It idles at around 75psi oil pressure cold, and about 30psi warm. But cold on the freeway it will run about 70-75psi oil pressure, and ~45 warmed up. But if I get on the gas a little it will jump up to 60-65psi. It has very good oil pressure, and the oil pressure gets higher as the engine revs more or has more load, so I think thats a good thing. If the engine is warmed up and the car is parked and I rev it up to ~2000rpm the oil pressure will go up to 50psi. I like it. At least it makes me feel that the engine is in good shape and is being lubricated well.
1990 Maroon Grand Prix STE McLaren 3.1 Intercooled Turbo, 14.695 @ 94.49 on 12psi before cam/heads/etc.
1990 Bright Red Grand Prix McLaren 3.1 Intercooled Turbo, 14.819 @ 92.72, 189whp
1999 Grand Prix GT 9.75:1|K&N|160 tstat|No Ubend/Cat/Res|UD Pulleys|Ported M90/TB/Heads|XPZ|2.55"|Pacesetters|Stage 2 I/C. 12.442 @ 112.78
2000 Blue Bonneville SSEi Supercharged Gutted Airbox|K&N|3.4"|1.9'*|WP Pulley|Custom DHP|180*stat|41932'*|No Cat. 14.681 @ 94.80
2002 Dark Cherry Grand Prix GTP Supercharged & Intercooled
12.084 @ 114.38 w/1.743 60', 315.6whp, Fastest Stock Cam M90 GTP!
NOW XP CAMMED, 340.6whp with XP, stock heads, Gen 3 M90, 11.984 @ 115.84 w/1.750 60'!
Most HP Stock Heads Gen 3 M90 GTP!
2004 Red Monte SS Supercharged K&N|3.4 Pulley|1.9'*|180*stat|41919 plugs|3" dp|No cat/ubend|DHP|Shift Kit. 14.2 @ 98
GOT BOOST??? Friends dont let friends drive stock!