Scenario: 2009 Silverado 2500 6.0, 95k miles. Previous owner had long used PYB 5w30 from a quick change place at regular 3k-5k intervals (have receipts). They used it to tow camper. When I bought it 5k miles ago, put in Amsoil SS (old formula with high calcium) and Fram Ultra. As of today, the SS is coal black. I was planning to run this first change-over to OCI 8-10k. Is it black from good cleaning or something else? Shall I change it out early, or continue running jet black oil?