I have a 93 with a single turbo conversion.

The engine has just been rebuilt is still in its break in period, only got ~400 miles on it. I just did an oil and filter change, and suddenly the oil pressure is spiking to the top of the gauge at 120psi or 8kg/cm2 if the engine goes over 4000rpm

The thing is, at idle, or under 4k rpm, the pressure reading just below 60psi, just like it was before.

I googled, and everybody always says it's the stock sensor/gauge that's the problem, but this seemed to happen right after I changed the oil,

I didn't overfill it. the dipstick reads full.

I used 10w30 penzoil, and I let the car warm up before driving it.

Any ideas?

 

Seems normal. IIRC, the Factory Service Manual specifies something like a minimum of 55 psi at 3k rpm. Fresh oil has no fuel dilution...something all rotaries seem to suffer from to some extent.
The only other thing I can think of is if the wax thermopellet was removed during your recent rebuild in favor of a plug. That tends to reflect higher indicated pressure. 86-11 Rx7 & Rx8 Atkins' Thermal Pellet (ARE50)