Just changed out oil from Motorcraft Synthetic Blend 5w20 to Pennzoil Platinum 5w20 in 2007 Kia Sedona 3.8L . The Kia is 90% driven in the 'burbs and only sees an Interstate for any length of time a couple of times per month . The MC Syn Blend was good for around 4,500 miles before getting dark amber with the tell tail smell of some fuel dilution (to be expected for OCI of mostly short trips ) . My question is what would be the starting safe OCI you would suggest for a switch to full synthetic Pennzoil Platinum 5w20 under the same driving conditions ...5,000 , 6,000 , 7000 miles ?