Oil Pressure

Well, I installed a new z-28 oil pump and rear-main seal in my Chevy 350. I also checked clearance on the rear main bearing and two of the connecting rod bearings. Rear main bearing was at .0015 and con-rods were slightly more, but well within spec. Fired it up and it made considerably more oil pressure at nearly 60lbs on the dash guage. I let it idle for a while, and the pressure dropped to 15 on the dash guage. On my way back to school, lots of freeway driving, my pressure returned to the normal 5psi at idle and just under 30 at speed. I am running a thinner 10w40 as opposed to the previous 20w50, so there is some improvement. So, if I am in spec on the bottom end and have a new oil pump, what could be causeing low oil pressure? Am I getting my oil too hot and I need an oil cooler? I turn 3300ish rpm going down the freeway and I think that would add a lot of heat. How hard is it to get to the oil pressure sender with the engine installed? I will try to find another pressure guage to compare against. Where would I install an oil temperature sender?

Getting lazy with your old age?

Oh, I did have excessive rod end play on the number 3 and 4 cylinders, could that cause low pressure? Or, is there a posibility of having a crack that allows oil to drain from the main oil gallery to the pan? Main bearings usually wear out before cam bearings?