I run Rotella T6 Full Synthetic 5/40 in my 2006 Ford F-350 6.0 Diesel. I had some engine work done and the shop added 2 quarts of Rotella T4 Conventional 15/40 because some oil was lost. I only had about 50 miles on the T6. Should I change it or leave it go until my usual oil change of 3000 miles.