I have a '11 Silverado 5.3 with 118k miles. Bought it at 113k and changed oil at 115k April 1 2017 with Valvoline Himilage full synthetic. I'm retired so low mileage and some short trips. When I OC at 3k on April 1 2018 (one year) last time, oil looked very dirty/black. (reason for OC, was going to go for 10k but looked too dirty, Fram synthetic filter) Would that be because of not running long enough to get rid of moisture? What would be recommended for oil type, conv/syn, OCI, and Brand for this level of operating? Very little oil consumption and engine runs strong, auto trans and 4x4 all stock.