Oil pressure

[Russel DeArman <russde_93@domain.elided>]

IH everyone, 
 got in last night from a beautiful weekend in the
Guadalupe mountains in west TX. Gorgeous scenery, and
pretty high (for Tx, 3000 foot peak, great hike).
 Question: I changed my oil before we left Friday
night. It was the first time I cahnged the oil in my
Scout (finally hit 2500 miles, figured I'd do it
before the long trip). 
  I put in 10W-30. My oil pressure is now doing funny
things. When cold the pressure will take a while (10
sec. or so) to come up to around 45 on the
after-market mechanical gauge (dash one doesn't work).
Once it's warm, at idle the pressure drops to <10, and
at highway speeds (2600 rpm's) it will only reach
30-35.
  I am thinking I should replace the 10-30 with 10-40,
but I am also concerned with the length of time it
takes to build pressure when cold. Should I
rebuild/buy a new oil pump? Can this be done
'in-truck'? i.e. without pulling the motor?
  Any help and advice welcome!!
TIA,

===
Russ D.
'73 Scout II 304
San Antonio, TX
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