overheating 95 Century 3100M V6

Hi,
I have a question about my elderly parents Buick. They have a 95 Century with a 3100M V6. This car only has 8-9 thousand miles a year of use. Recently (last couple of months) the car will go from center
scale on the temp gauge to just short of the red area on a moderate (and short 1/2 mi) highway grade during 70 degree weather with the A/C off or on. There isn't any coolant missing from the radiator or the plastic tank. The car has had this maintenance performed by the dealer:
oil & filter changed every 3 months or 3000 mi since new.
coolant changed (green stuff) and system bled of air every year.
belt changed three months before trouble started and looks to be in perfect condition.
the "service engine soon" light hasn't come on and no codes are stored.
no noise or leakage from the water pump.
new spark plugs 6 months ago.
radiator fans work.
pressure cap replaced with 16 psi Delco cap.
radiator and A/C condenser clean.
tires properly inflated.
brakes not sticking.
the dealer has never changed the original factory coolant hoses and claims they are in good condition.
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I had a car that started doing that and I changed to Castrol 5-30 Synthetic oil and it cured it. Just a thought.
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