I agree with Oliver. To be safe, I would get that oil out and put in diesel grade oil. I have been using Shell Rotella T conventional 15w40, the Valvoline Blue is good as is Mobil Delvac. If it were me, I'd run it for a tankful of gas, 150 miles or so, then use an engine flush that goes in the crankcase per the instructions and put in new oil. If the slipping continues, you may have a mechanical issue such as a worn out clutch, but I would not tear into the engine until I tried the proper oil.