I did a drain and refill of my engine crankcase today, and I thought that I had enough of Rotella T6 5W40 diesel engine oil, but I came up about a quart short. Went to 3 stores today looking for the Rotella T6, and all of them, the shelves were bare. Checked on Amazon, and no inventory either. What are your thoughts on adding a quart of Valvoline full synthetic 5W40 diesel engine oil to the Rotella T6 engine oil? T6?