I have a 02 HD2500 with 145k on it. I've always ran Quaker State 5W-30 in it with regular OC's. I'm looking at switching over to a synthetic was wonder if I should use a HM synthetic. Also considering the mileage would it be best shorten the OC the first time around. Will the detergent in the synthetic clean up all these years of running dino oil. I have no oil leaks at the present time.