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Do synthetic oils run hotter or
am I headed for cooling system trouble?
About a month ago I changed the oil from 15W40 to 5W40 Mobil One Synthetic. At that time the outside temps were in the 40 to 60 degree range. When I had the 15W40 in it, the scangauge showed the water temp going to 193, thermostat opening and the temp settled to 188 to 190 and never much higher. Oil temp is usually 5 to 7 degrees higher. This is based on driving at 65 MPH. Now with the 5W40 and it being 70 to 75 degrees outside, the water temp went up to 200 to 205 and stayed put, never higher. Oil temp was 207 to 210. This was also at 65 MPH.
I checked the coolant level, right where it's supposed to be, system has pressure, no residue around the cap.
Oil level is correct.
First thought was the oil and it being synthetic, holding heat, doesn't see right.
Then I thought the thermostat was stuck closed. It was replaced a year ago. If it was stuck close wouldn't the temps go even higher?
Then I thought, well, time for coolers? How could this happen with just changing the oil weight?
Then again, could it be normal? I don't think that's my luck.