My son has just replaced his original volvo with a used KAMD-42 that has around 2900 hours. The engine has been checked over, oil, filters etc all been changed. The engine starts and runs well had initially has an oil pressure of about 65 psi when under power. However after 10-15 minutes the oil pressure gradually drops to around 35 psi and then if you go back to idle the pressure drops back to virtually nothing and sometimes the oil pressure warning comes on. If you stay in idle for several minutes and then throttle up the pressure comes up to near normal and then slowly decreases. We are told the correct 15w-40 oil has been fitted. The engine runs at normal temperature around 85 deg with no overheating. Any suggestions, thoughts would be appreciated.