Well I just read your response. I used to get my toyota corolla model 1981 and 1984 both serviced from Western Motors in Karachi. Their mechanic used to put 1 litre diesel and one litre cheap oil for engine flushing. This he used to do only in case the drained engine oil was in bad condition or very dirty. I never noticed any bad effects whatsoever.
And I used to respect that engineer quite a bit as he had a lot of knowledge.
In new cars when engine oil is changed, the difference in drive is not that apparent, but in oldcars there was quite a lot of difference that you would feel on new oil change