What is "normal" oil pressure on Caravan?

Oil pressure recently spiked to over 100 psi with engine light going on and violent shaking of car (engine).  As I recall oil pressure was usually 40 psi...now it fluxates from 35 psi to 90 psi...hoving around 75 psi.  Found when accellerting slowy PSI would rise, but during normal driving and normal speeds pressure spikes.  Notice increase in psi during accelleration, but also notice fluxuations during flat driving and constant speed.  Dealer said computer read a one time misfiring of 1,3,5 cyinder, but have not been able to duplicate.  After driving a while psi settled down to just under 40 psi.  Car running smooth...35 psi lower than used to idle...stepping on gas while in neutral resulted in little fluxuation of psi for a while, then a sudden spike to 60.  I am may be mistaken, but I always thought psi remained at 40 during the last 2 yrs.  Water tempature at 198 to 201...not sure about what oil temp. should be...but your input would be appreciated.