like posted previously throw your ol plugs inn first and see if this helps, ive never ran thicker oil in my gtr but it is true that you could possiably be feeling more sluggish due to more friction in your engine, hence if you could run an engine with no friction it would preform alot better so glugging it up with a thicker oil to previous is going to make a difference, also not sure if i read correctly but "Also, the engine was cold when I tested the car, 5w (old oil) vs 10w (new oil) at cold." wouldnt recommend thrashing your car at cold engine temps, not a good idea, and if your oil pressure gage was reading low, your gonna need a new oil pressure switch (common r32 gtr problem) id recommend replacing this asap, as this will tell you honestly what oil pressure you are running, no oil pressure + faulty gauge could = BANG!
hope this helps