oil pressure question

i have a question for everyone, hopefully this has happened to someone on here before. last year we ran 60 wt oil in the motor. 14 quarts, black billet pump,tfx 92 block. we would start the car for the 1st time at the track, and everytime we would have 180 pounds of pressure on idle.......instantly. after about 3 minutes it would fall off 20 pounds or so. this year we changed rod bearings, same as we ran last year....just new. and all we had was 70 wt. oil, so i put it in there. this should give me more pressure initailly i thought, but when we fired it up we had 140. after about 2 min it climbed to 165, and stayed there. i know that is enough oil pressure, but can someone tell me why this oil pressure changed? what should i look at? main bearings? pump?
how did this thing change over the winter in the garage?

 

let me re-phrase.... it didn't matter where you started the car, in the garage, or at the track..........it was ALWAYS 180 on 1st startup of the day. no-one esle has ever had an oil pressure deal like that? i am almost ruling out the oil, maybe there could be something wrong in the motor? i dropped 2 mains tonite and they looked ok, will check the others tomorrow......this just really has me puzzled.