My truck has 385k on it.I bought it with 360 on it.It takes 6 seconds to build oil pressure.I'm guessing the oil pump is worn.How hard is it to replace in the truck.

 

Could be oil pump but it could also be the spring/plunger assembly in the oil filter mount assembly needs replaced.
The spring and plunger can be done by simply removing the 18mm bolt that sits next to the oil filter and putting a new set in.

 

I would check the pressure witrh an external gauge before doing anything else. It could be the pressure sensor or the oil pump. If you guess wrong, you will then have to replace the other device.