My 3.0 does/did(stock gauge is now unplugged) the same thing. Originally it only dropped that low when at idle, then started doing it on the highway when holding at a constant speed. I installed an aftermarket guage to get a better reading, the pressure is at the high end of specs when warming up, and is above 55psi when cruising down the highway.

10/30 weight should be fine for you year round, that is all I have ever used, and the weather temps the truck runs in range from 115 in the summer to teens in the winter.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed