Daughters Murray with briggs @Stratton 4 stroke oil black, will not keep running.

[Sp1nn3rman]

Lawn Boy, Honda push mowers. Daughters Murray push mower Quit running, I was able to start it but it just spits and sputters for awhile and then stops. Checked the oil and it was very black and had a smell of gas. It was not over the full level. Replaced oil and got it started again, ran for about 20 minutes (very rough), drained oil again, black again. Did this 3 times and still very black. Any advice about the black oil and sputtering engine? Sp1nnerman