When it is cold like below freezing my truck will sound alarm for low oil pressure when I start it up, and it seems like as I get out on the highway, and RPM rises it sounds again. Oil Pressure Gauge is showing just below 20 upon start, until I drive a few miles, and alarm sounds when I get over 45mph or so, until driven a few more miles. Eventually after I get moving 60 or so, it moves up to 40 or just below. I was running 5-30 Valvoline Max Life but changed to 10-30 max life on my three Brother in Law's recommendations. It did this before changing the oil weight, I also added a bottle of STP engine honey. Should I start thinking about a oil pump change? OR, should I purchase the oil sending unit, (or sensor?), Truck has 273,000 miles on her runs great no engine noises, but low oil pressure kinda scares me. Especially when I will be towing our 31ft fifth wheel camper.