automechanicaddictio

I am working on a 1987 Toyota 4x4 xtra cab pickup truck with the special order 3vze 3.0 v6 motor,it had extremely low oil pressure with coastal 10w40 motor oil & a ph3614 fram oil filter,it was registering between low & halfway on the gauge. I changed the oil to Chevron Delo 400 15w40 motor oil & a bosch 3330 oil filter,and now the oil pressure gauge registers halfway,with the motor hot or cold,what could cause the drastic increase in oil pressure,the different viscosity of the motor oil,or just the fact it was sitting,and i changed it twice already. Please advise whether or not i should run some motor flush through the engine after draining a quart of the old motor oil by removing the oil filter,then putting a fresh inexpensive oil filter on,then adding the motor flush and running per the directions. Sorry to be so long winded,i hope you can give me some advice.