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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.
Mine is the same and I to run 10/30 year round.

David Fritzsche
1990 Ex-Cab V-6,5-speed, with a few mods
04.5 CTD Dodge 2500 Ram--Tow Rig
Roseville, CA

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