[GTES-t]

As far as the engine you're saying it's an RB20DETT, is it actually a twin turbo? If so, it's not a stock RB20, which is single turbo. Or did you accidently put RB20 and mean RB26, basically, is the Skyline a GTR or GTS-t? Just trying to clarify the engine, if you have the RB20, it only comes factory with a single turbo and is the RB20DET. Now, it could be twin turbo if someone installed the GTR's exhaust manifold on it and twin turbo'd it.

Anyways, the max (recommended) PSI you can run on a standard RB20 is .9 to 1 bar which equates to ~ 13 to 14.5 psi. Now I've seen people that have boosted it up to 1.2 bar on a stock engine which is about 17.5 psi. However, it is highly recommended that you do not want to exceed 14 psi! Odds are your engine will run to lean over that and start melting pistons.

If you want anything over 14, you need to start changing parts on the engine... injectors, fuel pump and remap your ecu at a minimum! But to ensure your engine lasts longer and runs safer, add these to the list... Air Flow Sensor or add on signal convertor, fuel pressure regulator, upgraded/hybrid turbo and larger intercooler. Provided I din't miss anything, this should be able to take you up to ~18 psi safely.
I would also recommend installing a metal head gasket also, which is thicker and lowers your compression. You can get away without it up to 18psi, but definitely required if you go over that along with other internal modifications to the engine.