the oil pressure in our semi is dropping very low when the engine is very warm on on a hot day. Had a gauge put on it today and it has 16 lbs at idle but when it was very hot out it would drop low enough to make the light come on and shut the engine down. I believe about 10 lbs. The odometer shows 911000 miles and we have no prior history on the truck. this is the third year we have owned it and we probably put 5k miles ayear on it. Also the oil pressure is ok if we idle at 1000rpm instead of 600. Any ideas of what to do or do we assume its getting tired