You are not the only one that has a voltage meter that moves so far (SA63)... I have reciently changed my alternator to cope with my power hungry equipment in the car (Sunroof, A/C, Sound System, Subwoofer, Electric Antenna, Foglights [on when parking lights are on] Central locking and Alarm). I also changed the Battery... My Battry is 460cca and has enough electrics... When i drive it though the battery gauge ranges from 10 when idle (cold mornings) to 17 volts when running @ 120Km/h with the driving and fog lights on and sound system blaring. Hope that reassures you...
As for your Oil pressure Gauge... did you wire it up from behind your dash or from direct? If it is from the dash, resistance could be a result from old wiring chafing against something or loss in the ability to carry current accross the OEM wires (Just Guessing). My SA63 Oil light shows every week for me because i burn as much petrol as well as oil .